Unfortunately, to ask someone how to play piano isn't a dissimilar question. =P
What I mean by that is that there's no concrete answer that Ganon has to characters like ZSS. Her best keep-away tool is obviously her stungun, but if you PS that, you run the risk of getting grabbed, so you have to guess from there how to act. As I said in another thread about approaches, the best thing to remember as Ganon is that your approach doesn't need to be lightning-speed.
Use empty jumps, powershields and so forth to get close to her and anticipate grabs. Most of the time if you're having difficulty approaching as Ganon, the only viable answers are case-by-case, since it involves a lot of creative movement and bluff-calling. Fr example, attacking her shield with FAir to condition her to shield so you can mix up with something.
When Ganon pushes your to the corner, you're forced to act, so slowly and deliberately make your way to her. Throw our DTilts and UAirs to discourage any sneak attempts at sneaking behind you. Takes iron patience sometimes, but it's the best we've got, I'm afraid.
Edit: Wizkick's a good choice for entering, too. Though, you have to be choosy about when you use it. Force out a spotdodge, roll, etc. with it. It also clashes with projectiles, so it's a good way to cover ground if you're feeling saucy enough to cope with being put at a potential slight disadvantage upon canceling the projectile (if their projectile ends shortly before your wizkick does)