Samsung has been working on a dedicated Dolby Atmos gaming mode for the Android 9.0 Pie update for its 2018 flagships, the Galaxy S9/S9+ and the Note 9. The mode has already gone live on the S9s in previous Pie betas, but is not present in the latest release, as spotted by SamMobile. The option is, however, live in the Pie update for the Galaxy Note 9.
The Dolby Atmos gaming-focused mode is its separate thing, instead of being nested with existing presets like "Movie" and "Music." This means that you can enable it only when you need to, without having to change your main Dolby Atmos preferences every time you want to play a game.
As far as actual functionality goes, game mode tries to enhance the sound by making the surround effect more pronounced and increasing the overall volume. If you're running the latest Pie release on your Galaxy Note 9, you can enable the option from Settings.