When Ganons are zoning against marth they tend to do fade back Fairs so in order to beat that you can over extend a nair which will lead into a juggle situation depending on the DI.
Also Ganon is sloooow so when you get grabbed it's almost always advantageous to DI their Down throws behind them and then tech away. Don't get predictable though, when they catch on you can mix up DI behind to tech inward.
Marth's use of the Dtilt as a zoning tool in this MU is diminished due to ganon's slow ground movement, but be wary of those ganons who love to waveland towards you into jab or grab or whatever because an anticipatory Dtilt shuts that strategy down.
Up throw is great to start a juggle situation as long as you are patient and watch to see how they use (or don't use) their Double jump.
I hope this helps a little.