If you have a "launcher-type app" (a regular macOS app bundle) which is used to launch the actual game executable (a non-bundled executable, such as a Java game), Game Mode does not seem to work as desired in this case. If the launcher app has the appropriate LSSupportsGameMode and LSApplicationCategoryType set, the launcher app will activate Game Mode if fullscreened but the actual game will not (FB18026957). Is there a workaround for this?
It also seems like the system hardcodes some apps to act as "game launcher apps" which do have the desired behavior - for example, if you have a sample app set to Steam's bundle identifier (or just use Steam to run the program) and have this architecture where it runs another executable which is the real game, then that game can inherit Game Mode, which is also supported by system logs in Console (note the "launcher" labelReason):
default 17:11:05.008533-0400 gamepolicyd Found game GameProcess(Optional("java"), pid=33754, euid=501, labelReason=launcher, launchedBy=Steam)
Is there a way to opt into or apply for this behavior for our own apps?
I previously asked this on the forums but I'm also wondering if any new developments have occurred since then.