The Gigantic would probably reject Aptom, similar to how the Guyver armor would probably reject Aptom if he was simply covering Sho. Thecontrol medal is a bit of a dick that way.
What you seem to not realize, is that Aptom was only covering Guyver 1 like a Coat. Guyver 1 gained no new powers. Aptom gained no new powers. They just looked mighty cool.
So if Aptom did or was able to cover the Gigantic, other than possibly looking cool, the Gigantic would not gain any new powers, nor would Aptom. Aptom would just be able to use his current powers to conceal the Gigantic, freeze things, or provide fire support from beams and missiles.