In the air, small gold nuggets ID in the 35 to 50 range. However small gold nuggets are weak signals and when the nuggets are in the ground you can't depend on them to ID correctly.
Gold prospecting is almost always done in all metals mode. A good strong hit that registers consistently in the iron range on the "speedometer" is probably shallow iron and can be ignored if your experience on that site (after having dug some such hits) validates that. Anything else is a "digger", and your best discriminators are your eyes and a good magnet.
Just about the only situation where you'd use discrimination mode for gold prospecting, is on a promising site that's strewn with iron trash, and you accept that if there's gold on the site you'll be missing some or most of it due to iron masking and the discriminator itself.
If you're new to gold prospecting with a metal detector, I recommend that you read the book "Gold Prospecting with a VLF Metal Detector", on the TekneticsT2 website. Or your dealer may have a print copy.
--Dave J.