The best way is, as you already figured, to find out which modules actually clutter the screen in games and only hide those when entering gaming mode.

Usually all visible modules have specific bangs for hiding them, for example !SystrayHide, !VWMHide, etc. Go through your modules for all that you want to hide, search your docs for the appropriate bangs and !execute a series of bangs instead of the !HideModules or whatever you seem to be running now.

An even better option could be to just get rid of module transparencies and OnTop -statuses for the gaming session, because those are the real culprits. However this is more complex to achieve, and depending the modules not always possible.