But why? What reason is there to call him Gannon when it's clearly not his name? It'd be like calling Zelda "Zedla" or something, or Link "Linkk"... Sure, nothing's stopping you doing it, but... seriously what's the point? Expect the vast majority of other fans to get kind of annoyed about it...
^ FE said it all. I just have a little more proof to add.
Here's an extract from the official Japanese manual that came with the game... even in the early Japanese it is Ganon... the only time it was spelled otherwise was in the PAL/US version of TLoZ... which was an error and has been since...