I might add "GroundScanner" that it always pay to go slow and work into the more advanced settings such as zones and settings such as tone volume. So may I suggest getting familiar with the machine by simply using the 3 tones on start up, run the machine over the nearby ground, looking for a totally "clean" spot to ground balance, and then GB, -being especially careful that you have a perfectly clean spot free of any metal- (this is a very common mistake) and THEN if still quiet, -bring up the GAIN-.
The default gain is quite modest, and again -if the machine is running quiet, with only a small amount of background noise, and EMI is not an issue at your working site, go ahead and bring up the gain to even 80-90. It's all about familiarizing yourself with the settings. Stand there at a target and bring the gain up and down and listen to the huge difference in the sound. The signal tone will be much cleaner and over a bigger area with more gain. Of course you realize that with greater gain, your chances of locating deeper or smaller targets also increases. Ground balancing correctly and in the different modes of doing GB...is very important. That is why the manual gives two pages on this.
It's also -good practice- to also do a couple of GBs during your session. And of course if you change your site, GB at once! Running all that gain may not be possible for your general location. Use gain judiciously...but again practice with different targets and levels of gain. As others here pointed out... there is no set group of settings. It completely depends on your soil, your preferences, the coil you have on, even how moist the soil is. You'll laugh...but this is very good advice. Ready? Leave the owners manual in the bathroom and read it cover to cover about four times. Every time you read it through...you will glean more good information to help you. I feel the book is well written.
Good Luck and Merry Christmas!