I believe I read something about that, recently. Apparently, the Sound Blaster emulation on the GUS is more or less "hardware-assisted". Yes, SBOS is a software emulator, but instead of relying on I/O port trapping (which would require at least a 386 and wouldn't work with protected mode games), the GUS listens on the Sound Blaster and Adlib ports and provides a mechanism for SBOS to receive and process such I/O in software. This is why SBOS even works with most protected mode games. Also, I'm not entirely sure, but I think I also recall reading that this functionality was only offered in later revisions of the GUS. If true, then ironically, you might be better off with an older revision of the card, if you want to use it together with a Sound Blaster. Also, Gravis later released the Ultrasound ACE, which was intended to coexist with a Sound Blaster (or compatible) card. But of course, that's not of much use to you with your current setup.

Perhaps there's a way to turn this feature off on a regular GUS, like yours?

Let me do some Googling to find the documentation I recall reading about this.

This is what I found in the Mega-Em FAQ:

QUESTION: Could Mega-Em be modified so it does not require a 386 memory manager, and hence work with software such as […]
Show full quote

QUESTION: Could Mega-Em be modified so it does not require a 386 memory
manager, and hence work with software such as Comanche,
Ultima 7 etc. ?

ANSWER: No. Mega-Em requires 386 protected mode for it's emulation to
operate. It uses the memory manager as a means of entering
protected mode, while still remaining compatible with a wide
variety of software. Mega-Em can not work with the above
mentioned software because they can not coexist with other
protected mode software.

QUESTION: Well how come SBOS works with the above mentioned games?

ANSWER: While SBOS is indeed a software emulator, it is hardware
assisted by specific features built into the UltraSound card and
makes no use of any protected mode features of the processor.

Source: ftp://retronn.de/driver/Gravis/UltraSound/ULT … 3.04/MEGAEM.TXT

Also, I was apparently confused about this feature only being present in certain revisions of the GUS. That apparently only applies to Roland emulation in particular, not Sound Blaster emulation, as can be read in that same FAQ:

QUESTION: Mega-Em gives me a warning saying I have a pre 3.xx UltraSound board and Roland emulation will not work with […]
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QUESTION: Mega-Em gives me a warning saying I have a pre 3.xx UltraSound
board and Roland emulation will not work with most protected mode
software. What gives?

ANSWER: Mega-Em is supporting your UltraSound card as best in can. However
the early revisions of the UltraSound were never meant to emulate
Roland sound devices and do not have the necessary logic to
provide protected mode emulation. Despite this Mega-Em will
still give Roland emulation for real mode software.

Hey, here's something you could try: configure your Sound Blaster Pro card to work on a different I/O port. So not the default port 220, but 240 or something. Then configure your game to use the Sound Blaster card on that port and check if that works together with your GUS. I know it would still suck for older games that don't allow you to configure the Sound Blaster port, but at least that would be something.

I am going to use GUS MAX + SB Pro 2.0 in this build - Nexgen - the 486 killer - WIP

I read somewhere the trick is to load SBOS and then unload it in the AUTOEXEC.BAT. Then the GUS will no longer handle the AdLib port range and the SBPro should be unblocked.

Well, I've used a GUS Max and SB Pro 2 side-by-side for ages, and on my system AdLib always worked (never used any SBOS trick), so I'm not really sure what's going on here.
As far as I understand it, the GUS may indeed have some hardware-support for SBOS/Mega-em, but it is disabled by default, and only enabled once the TSR is loaded.
I've wired the cards up with a switch.
One way the SB Pro 2 is base 220h and the GUS is 240h, and the other way, SB Pro 2 is 240h and GUS is 220h.
Never put both on the same base address obviously.

I have a 2.x GUS (not home so can't check exactly which) and definitely can use FM on other cards in the system - I run it with PAS16 and NXPro. Can't recall which of those two does FM (both OPL3 anyway) but definitely not the GUS.