The Gate is a standalone game which uses Xash3D FWGS engine.
THIS GAME IS NOT SHIPPED WITH GAME DATA. YOU MUST GET IT FROM MODDB
To play it, make sure you followed these steps:
1) Installed Xash3D FWGS
2) Copied "valve" folder from your Steam account to your Android device. (e.g. to "xash" folder on your device storage. If you choosed another folder, make sure you are choosed this in Xash3D FWGS app)
3) Copied "TheGate" folder with game data downloaded from moddb to your Android device.
Mod page: http://www.moddb.com/mods/the-gate
4) Installed this app from Google Play.