If the AT Pro was as good as an AT Gold for prospecting then Garrett need not have made the AT Gold. If you compare features you will see you give up almost nothing going to the AT Gold over the AT Pro. You get a true threshold based all metal prospecting mode the AT Pro lacks. On the flip side the AT Gold has a smaller coil (good or bad depending on how you look at it) and the custom saved notch mode on the AT Pro is missing on the AT Gold. The AT Gold higher 18 kHz frequency is not as well suited for salt water as the AT Pro 15 kHz, which still is not that great.
Any metal detector can find gold nuggets if they are shallow enough and large enough. That said nugget detecting is one of the most difficult types of detecting you can undertake, and any advantage offered by way of equipment is desirable if you wish to be successful. So yes, I know I could go find gold nuggets with an AT Pro but it would not be my first choice. If you are serious about metal detecting for gold nuggets, get a detector which was designed with that as its primary design goal.