I wouldnt use and MG Gyan for that. The Gyan is a massive suit. It stands at 19.9 meters, and Tallegeese at 17.4.
Even the Gundam and GM are taller at 18 meters.
I think you'd be best getting an MG wing, as its much shorter; around 16.3 meters. You'd be able to cover the frame with the Tallgeese parts and not have to fill as many major gaps comming from scale issues. You know, smaller things fit into bigger thing much easier than vica versa.
I think with a conversion like that you'd want things to be as easy as possible :)