To be fair, both omnipresence and immeasurable were hardly portrayed on-screen for Gaim. However, it should take in consideration that both power were an part of established lore of the Golden Fruit and we cannot ignore it because inconsistent term. Let me lay out something here:
Kiwami Gaim & Ryugen VS Jam: This fight is clealy one-sided given that Jam barely did any notable damage to the heroes, it's safe to assume that Gaim obviously went easy so that Micchi could went through a character development atone for his sins and all.
Kiwami Gaim VS Megahex: Megahex was consistently shown to be much stronger than Gaim early on, even taking Kiwami Lockseed from him at one point. Plus, he is a one-off character so all his feats is neatly tied in one movie.
Movie crossovers like
Heisei Generation Final and
Dr. Pac-Man vs. Ex-Aid & Ghost with Legend Rider however, they seem to equalized his power on par with other Riders so he would not stomped everybody, as with Drive and Wizard.
In Zi-O though, i think he wanted to have a normal conversation with Sougo because why would he do otherwise?
For Decade, he don't have Gaim power at base so i have change to Complete 21 instead. For Grand Zi-O, i believe he only has Kiwami power by touching the engraved symbol on the armor. Technically, both of them need to activated Gaim's abilities first as they don't blitzed anybody right off the bat.
Ohma Zi-O, like you said, never going all out in all his appearances and that one time Trinity caught him off guard might be outlier because he was trying to teach Sougo about path of king rather than outright kill him.