This is a red flag in my book.  Even the 'default' of 10 might be too high, depending upon the conditions.  I have no experience or knowledge of the Garrett ATX but very few detectors can be run at max gain in the gold fields (and in fact many can't handle it even in parks/etc.).  Have you watched the video below?  Bill Southern and Kevin Hoagland make a big deal about not setting the gain too high.  (They also talk about properly setting other things, such as threshold.)  I'm sure you'll get so