Even if you suspect no issues with your pack, many feel it's a good idea to exercise the pack perhaps twice a year to insure it's at top capacity for longer run times. I do this with all my packs and it will both keep them in tip top shape and also increase the capacity. Some people think only Nicads suffered from memory problems. While that's true, and in fact isn't really an issue among Nicads these days as far as I know, Nimhs will still benefit from giving the chance to exercise their muscles and stretch their legs a few times a year, so to speak.
The first step in exercising a good pack for more capacity, or in trying to recover a bad pack, is to drain it completely dead. While my charger has a discharge function, it's limited by the watts it can handle (voltage of the pack X the amp rate of discharge). For that reason, I prefer to drain the pack on a car tail light bulb because it's much quicker. After the bulb has gone out let it sit for about 20 minutes to insure it's drained every cell to equal discharge. Also, be sure to place the bulb on a cement floor because they get very hot, which you should always do anyway with your packs when charging or discharging in case a fire should happen just to be safe.
Now you want to charge it the first time using a more "aggressive" charge rate for the reasons being discussed shortly here. You can exercise or try to recover a pack with the stock charger, but if you have an aftermarket charger then the first charge should be perhaps around 1C. For the stock GT pack which is 1000ma, that would be a 1 amp charge rate. This will bust down any crystals that form resistance and are not normally destroyed by a low amp rate to charge the pack. I have even done this 1C charge rate (roughly a 1 hour charge time) when in a hurry to get to the field when I still used the stock rechargeable pack, but I feel for the life of the cells it's always best to charge at a lower amp rate, like perhaps 1/2 to 1/3rd C. A 1C charge rate won't shorten the life of most modern nimhs or nicads, and probably won't with the GT pack either, but I would by all means not make it a habit in order to play it safe for maximum life/capacity of the pack. Being in a hurry to get to the field here and there probably isn't a problem though.
But for exercising the pack (even if it doesn't appear to be having issues or problems with short run times or just being a bad pack), I'll do 1C for that first cycle to break down any crystals that can form over time, but some will go as high as 2 to 5C to insure those crystals are broken down, at least in nimh or nicad packs for other applications such as in the RC plane/car world.
If I don't know what C charging rate a pack will handle, and yet I feel it's not working properly due to crystals, only then will I risk a say 2 to 5C charging rate for perhaps 2 to 5 minutes and that's it. Charging at such an excessive rate on a pack not designed to handle that can cause risk of explosion or fire, but if I feel it's needed as the only hope to recover a pack I'll risk it for that few minutes, using proper safety gear such as goggles and such of course, and then will return it to a 1C charge rate for the rest of the charge, but still will feel the pack during that charging process to insure it's not getting hot. If it's hot then you are pushing the pack and heat will slowly destroy the pack's useful life over time. Either way, if I don't suspect the pack has issues then 1C is what I'll charge it at for the first excises cycle, and even then it's a good idea to feel the pack while charging to see if it's getting hot.
So once the pack has been first drained, and then charged at that first "aggressive" rate to insure any crystals have been broken down, then it's time to drain the pack again via the car tail light bulb.
IMPORTANT NOTE HERE: If you do use a car tail light bulb make sure you are not dead shorting the pack. Some car tail light bulbs have numerous contacts for various functions. If you have any doubt about this stuff then best to find a bulb with only two contacts, such as a side blinker signal or something. While some will drain nimhs/nicads via dead shorting them, I feel this is a very dangerous practice, or at least is very hard on the cells IMO. They have to have a load to drain them, such as a 12V light bulb of some type. You just want to be sure to use a light bulb that will drain the pack fast and won't have you waiting for hours. A typical drain time using a car tail light bulb might be perhaps anywhere from say 10 minutes to an hour or so, depending on the capacity of the pack and the wattage of the bulb. As said, once the bulb goes out let it sit for about 20 minutes at least to insure the cells are all at equal discharge.
And again, make sure that bulb is on a fire proof surface because they get very hot.
Now it's time to charge it again. Now I prefer a more gentle rate. The stock wall transformer will work fine but it's going to take at least 10 hours or more to complete it due to it's super low trickle charge rate. Using an aftermarket charger, I will charge at say 300ma (about 3 to 5 hour charge time) to say about a 600ma charge rate (about 2 to 3 hour charge time). I don't feel either of these is really pushing the 1000ma GT pack.
Time to discharge again. Do this drain/charge exercise at least 3 times, but probably about 6 will yield maximum capacity and get the pack into it's best shape possible. It helps if you are using a charger that displays the capacity put back into the pack, because you can visually note if the capacity is still making substantial gains with each charge. When I don't see the capacity increasing all that much, I'll still do another charge or two to see if it in fact has hit the wall in terms of it's maximum capacity ability.
Remember too, that even if the pack seems fine, by exercising it you are going to increase it's run time. Also remember that a new pack also needs such exercising to get it into top shape. Expect very short run times from a pack until it's been charged a few times. Do the exercising (3 to 6 times or so) at least once a year or perhaps twice a year to keep your packs in top form.
There are some who believe a nimh or nicad should never be drained completely dead or you risk a cell reversing polarity, while others (like me) believe it's important to drain them completely to insure full exercise of the pack. From what I've read the reverse polarity risk is mainly when the pack has set on the shelf after it's been completely drained without re-charging it. Besides, even if you let the pack sit hooked up to a light bulb for 20 minutes or even an hour after the bulb has gone out, you'll find that when it's removed from the load (light bulb) the pack will immediately start springing back up in voltage. It would have to sit hooked up to a load for hours if not days before all charge is truly gone from the pack.
We aren't trying to reach that point with the light bulb, we're just trying to get all the cells at equal discharge to insure they all reach full charge at the same time on the charger. With cells out of charge balance with each other some might peak and cause the charger to believe the pack is fully charged, when in fact several cells might be while others are not there yet. That's one of the things that can cause a false "surface voltage" or "surface charge" of a pack, where it seems to show a high voltage when not under load but quickly drops when any kind of load is put on it.
Oh, by the way, if you are wondering how to hook up your battery to a car tail light bulb, head up to Radio Shack and get a pack of connection wires that feature alligator clips on them for about $3. Simply hook up one lead to one main battery lead on your battery, and it's other end to one of the SEPARATE connections on the light bulb. Then do the same for the other alligator connection wire. The positive lead from your battery should never be connected in such a way as to be directly passing current to the negative lead on the battery. That's a dead short. You always want a load in between them for draining, such as a car tail light bulb.
I think I've pretty much covered the exercising or trying to recover a bad pack thing as good as that's gong to get, and since this thread is now linked to the battery sticky in the future I'll just point people to that who have any questions on exercising a pack or trying to recover a bad one.
Once again the warming: Don't follow my advice or risk personal injury, death, burning your house down, destroying your detector, cooking your battery, or anything else bad you can think of. 
I make no claims to being right about all things. Only God can do that. And besides, opinions on the care and feeding of rechargeable batteries differs greatly among people. These debates have been going on for decades even about nicads, so do your own research and make up your own mind. I just am basing my opinion on my own research along with my own personal experience with rechargeable batteries for years in various applications, hobbies, and so on.
