Installed games from the Play Store are usually added automatically to the Game Launcher, but in some cases games are not detected, but can be added manually.

Furthermore, normal apps can also be added to the launcher to use the functions.

Android 9 | LG UX 9.0
Step 1:  Open the  Game Launcher
Step 2:  Open the  Menu
Step 3:  Tap on  Add as game
Step 4:  Select all desired  Apps
Step 5:  Confirm with  Add
  1. Open the Game Launcher
  2. Open the Menu
  3. Tap on Add as game
  4. Select all desired Apps
  5. Confirm with Add

LG Instructions