This covers details about GBA versions of Doom I and Doom II.
Background[]
There is no specific dates related to when the game takes place in the game. It's not known if it goes with the 21st century idea seen in the classic novels or SNES doom release.
The main notable detail is that it goes with the split between the Marine from Doom 1 and Marine of Doom II being two separate individuals. With the first reaching earth via the teleporter from hell at the end of the game. While the Marine (Doom II) came to earth via Drop pod after fighting demons during the invasion of Mars Base, and fought through the underworld of Mars.
In general it mostly follows the original Doom timeline introduced in the original manuals, which had the similar setup.
There never was a release of Final Doom for the GBA, so its not clear how it would have incorporated things.