Bethesda Software, the owner of id Software, published the Doom 3 BFG source-code to a new GitHub repository. Like earlier id Software code drops, the game's data remains proprietary so you must purchase the game still, but the engine is fully opened up under the GNU GPL.
Doom 3: BFG Edition is a re-release of Doom 3 that happened last month. Doom 3 BFG introduces better graphics, better sounds, a check-point save system, support for 3D displays, and many other new features since the original Doom 3 game release in 2004. Doom 3 BFG Edition, like the original Doom 3, is based upon the id Tech 4 engine.
There is no official Linux client for Doom 3 BFG nor are there any plans for such a port, but with the availability of the BFG source-code plus the existing Doom 3 / id Tech 4 source-code that has Linux support, it should be easily possible for the open-source community to bring the BFG Edition game natively to Linux.
Aside from pushing the Doom 3 BFG source-code to GitHub just minutes ago, they also announced the source availability via the Bethesda Blog have also also announced a game update too.
While the Doom 3 source-code has been available for one year, there aren't many open-source game projects yet using this id Tech 4 engine. Hopefully with the improved BFG code now available, there will be renewed interest in ioDoom3 and other similar projects.