These are the points you have to address:
- You need to prove that SWGB contradicts the movie substantively. Oh, by the way, it does not. I've played it start to finish. The only non-canonical elements are clearly stated to be post-facto Trade Federation simulations.
- You need to disprove the existence of the Mantis, this is probably your best shot, as you may be able to get the OPer to clarify the exact date. Even so, this does not disprove the existence of any other gungan Starships. It would be exceptionally implausible for them not to have access to starships; the Naboo certainly did.
- You need to come up with something more than your own word to say that the gungan theatre shield will not stop physical projectiles, given the explanation that objects with ground contact can be earthed to pass through.
When you can address these points, without squealing that 'It's not canon unless it's in the film, which I've probably not watched for years' we'll talk more.
I will not reply to any efforts to argue that game units are not Canon, as Hoth has clearly explained for you, the Star Wars canon is inclusive of anything that is not explicity and directly contradicted - if the game said the Gungans were all hot pink, that would be out. If it shows other pieces of Gungan technology however, that is canonical.