First and unfortunately, I do not own a GUS. But I got the IDE-controller on my SB16 boards to work fairly easy.
I don't think the GUS will require much different handling, basically you'll need two things:
1. IDE driver for GUS. Originally one came with your card back in the nineties, but I'm sure the Vogons driver archive will supply one, too. Maybe as part of the full driver-disks archive.
2. MSCDEX (or equivalent) CD-Extender.
When your system already sports two IDE controllers, the soundcard-IDE controller must be set to tertiary.
With Soundblaster cards that is achieved by loading the IDE driver in config.sys with the corresponding argument.
My SB16s all work fine as tertiary controller with this line in config.sys:
DEVICEHIGH=C:\SB16\SBIDE.SYS /D:SOUNDLW01 /P:1E8,11
SBIDE.SYS ist the IDE controller driver, SOUNDLW01 is the drive's name (choose one freely, but supply the same name to MSCDEX in autoexec.bat, too!) and P:1E8,11 sets the SB16's IDE controller to tertiary.
Mayhaps someone here could tell about the corresponding tertiary settings and driver filename for use with a GUS?
My SB16 manuals provide all these informations. I dare to think, that Gravis would have in their manuals, too. Do scans of the manuals exist (havent looked for them, tbh) or does one still have his original manual to look that up?
Hoarding the precious, worshipping the ancient, playing the forgotten.