From a quick listening I can't make out any differences at all.
Anyway, I've noticed that Grabbag uses the Power drum kit. Additional drum kits were a feature of GS... that fits with my experience with the Yamaha where the default TG300B/GS mode plays stronger drums for that track than the GM mode after starting Doom. As in, the extra banks aren't available in GM and it just defaults to the standard drums. But then I don't get why this track seemingly doesn't show the same behavior on the Roland...?
Ah, by the way... does the instrument display on your SC-55 change to where it has a _ between the instrument number and the name after you start Doom? It does that on my SC-88 when manually sending a GM reset, even if I couldn't find anything that would hint at it in the documentation.