After trying out 19 media players (and a lot of unexpected delays), I found 4 that will load GUS patches and play MIDI files in Windows95/98.
Gravis WinDecks 1.12


Homepage: N/A
Download Link: http://dk.toastednet.org/GUS/utils/WD32v112.zip
A media center from Advanced Gravis, can play MIDI, wave, tracker files and Audio CDs. Very easy to use. After installing, read the help file for more details.
Clarion 0.9.4

Homepage: http://www.geocities.com/SiliconValley/Heights/9860/ (offline since late 1999, use The Wayback Machine to access it)
Download Link: https://www.dropbox.com/s/bs1q744gt6gtzys/Cla … ion094.zip?dl=0
Plays MIDI files and Audio CDs. Freeware, easy to use but not very stable (crashes frequently). Since it has a common name it's very difficult to find a proper download link, so I provided my own via Dropbox. I'm also aware that a 0.9.5 version was available at one time (corrects some networking issues), but it's almost impossible to find now.
MIDI Genie 1.90

Homepage: http://earsnest.tripod.com/
Download Link: http://members.tripod.com/~EarsNest/midig190.zip
Plays MIDI, wave, avi files and Audio CDs. Used to be shareware, now it's freeware - the homepage has the info needed to register it and a list of features. Easy to use. Only one of the download links works on the site, I decided to list it here to avoid confusion and frustration.
WinJammer Shareware 3.0

Homepage: http://www.sonicspot.com/winjammer/winjammer.html (offline since 2015, use The Wayback Machine to access it)
Download Link: http://slsite.tripod.com/WinJammer_shareware_install.zip
WinJammer Shareware is a MIDI sequencer for Windows 3.1, 95, 98, and NT. It can record, play, edit and create standard MIDI files. A companion program called WinJammer Shareware Player is also included, can play MIDI, wave and avi files. While the sequencer is somewhat involved to use, the player is easier to understand - just make sure you read the help file first after installing. Unfortunately it is shareware, and registering it now is apparently impossible (the site went offline in early 2015); this is not a major obstacle though, all I will say is that Google is your friend in this case (and before anyone questions my motives, I am a registered user since version 2.30 and no, my personal code is not out there to be found 😜). The download link is for the 32 bit installer only.
I'm sure there are more Win9x media players out there that can load patches and play MIDI files with a GUS, these are just the ones I found in my own collection. Windows 3.x media players could also work, but they have nagging problems of their own - no long file name support and 16 bit code that's not suitable in a 32 bit environment are just two that come to mind.