IMO, the only real complex part of GG is its approach to frame data.
for one, the game has a mechanic that lets u decrease blockstun,
next, when attacks hit they have properties that say like "on counter this move will cause X frames of Y type of stagger" which of course opens the window for your character to capitalize on and do a combo.
BUT! Tekken does the exact same thing so it's really not too difficult.
as far as Roman Cancel system goes, Street Fighter 4 does a VERY similar thing with Focus Cancel so if you couldnt figure out roman cancel, get ready to go thru a similar frustration when learning SF4. Garou had a similar False Roman Cancel system also as certain moves could be cancelled and followed up with supers or combo extensions. I also thin Tatsucap's Baroque system is similar also.
Now, the thing that makes GG good is its balance and its uniqueness of characters. You can look at the whole game system and mechanics when in reality you really do not need to know like a large handful of that. When you play GG you play your character, you learn your character and when the game has a system mechanic that your character can take advantage of then you implement that into your playstyle.
for example, I use Axl and I dont care about Impossible Dust becoz I cant set up jack shyt with it and also coz my standard dust combo does good damage. I also dont need to use Jump Install except for those one-time situations when you want to do something that your opponent is not expecting, and if you play the game enough you'll already know how to do it.
in a game like 3S, you can succeed with Twelve or Sean, but you cant play them as a unique Twelve or Sean... you have to play them in a way that takes advantage of the same 3rd Strike system that each character is also taking advantage of. In GG, your strength is solely determined on how you play your character.