Set the default GPU device ID to 0

[Calinou]

This avoids confusion with the default configuration value being gpu_device=0 (as commented out in the gamemode.ini example), but the actual default value being -1.

The alternative is to document this behavior and set the commented out value to -1, but this makes the setup procedure more complex. The new default behavior suits the 90% use case where there's only one dedicated GPU available.

See https://github.com/FeralInteractive/gamemode/issues/351#issuecomment-1278251075 where I stumbled upon this issue today.