when you use the GUS PnP Pro in "GUS classic mode" for DOS games compatibility, does the sound card use its built-in 512k RAM to store and use the "GUS classic" patches?
Yes. It is benefit of Pro
variant in comparison with regular PnP card. It have full Classic/Max compatibility right out of the box without adding extra ram.
If you are running a DOS game using a windows 98 "dos box", will the sound card not take advantage of *any* of the extra added RAM banks? If this is correct, will the GUS PnP Pro always sound inferior to a GUS Classic card with 1MB of RAM, when playing DOS games?
Yes. Until game not support Interwave directly it always sound right like GUS Classic/Max. "Big" PnP's memory can be used only in Interwave supporting games and apps (very few), or in General MIDI emulation (not sure if it works under Windows DOS box). But you always can replace original GUS patch set a.k.a "Ultrasound Compatible Patch Set" in PnP's interpretation by another one patch set like "Pro Pats" or "Pro Patches Lite".
Here you can get some GUS ISO's:
Original installation CD for Classic/Max/Ace
Same for PnP. Later re-issue (by 1996) with 4mb Patch Set on it.
Legendary Utopia Soundbank CD-ROM. Unfortunatelly not original, but re-assembled package with some ancient warez added. This "version" came from ex-USSR fido groups in later 90's. Even in such non-authentic look is very rare peace of software.