The GPL license only covers the game's engine and not much else
DOOM 3 BFG Edition was officially released for the consoles and the Windows platform more than a month ago, but id Software hasn't forgotten the Linux community and now they've also published the source code for the game.The initial announcement was made in October by John Carmack on Twitter, but there was no other indication of the release date.
Bethesda and id Software have now published the source code for DOOM 3 BFG Edition on GitHub, under a GPL license. This means that the game engine itself is free, but not any of the art assets, including the music.
Before getting your hopes up for a DOOM 3 BFG Edition native client for Linux, you should know that source code for the original Doom 3 has been released for years and there is little to no progress made on a Linux port.
If you are interested in the source code, you can find it on id Software GitHub page.