You can use her ignorance of Ganon's capabilities to create openings, and then make the best of those opportunities. Every matchup in this game is winnable.
basically this.
so ganondorf vs sheik. this is how i see it.
don't use fair much unless you're 100% sure it'll hit. otherwise you're probably grabbed after.
space with bair and titls/jab
if you think they're going to dash attack (bad sheiks will use dash attack at pretty ****ty times) you can time full hop dair and it'll **** em up real well
practice cg like crazy. don't drop cg vs sheik. play gay as ****, etc.
when sheik is above you, just spam uair (well not really but yeah) like crazy cause wtf is she going to do, seriously.
and then for edgeguarding, just ledge grab stall and time it so that sheik will go into a bad recovery position ( sheik up-b is super vulnerable on stage)
that's all basic matchup stuff for bottom/low/mid tier sheik players.
high/top tier sheik players are a completely different story.
matchup is definitely winnable if you can surprise your opponent/get good reads and mixups. look at it like this: how many ganondorfs are there in the world that sheik players get to practice against? surely there are more sheiks available for us ganondorf players, so we should basically be aware of all the lame, wack **** she can do and we should be able to adapt accordingly.
use gimmicks sparingly, mix **** up, and don't get greedy. the matchup will feel a lot easier, i promise.
and no pride ****. i can't believe any ganondorf wouldn't cg vs sheik. pretty bad idea imo.