Just in case you can stand having Steam DRM installed in your system, I recommend taking a look at Strife: Veteran Edition. Strife is a Shooter/RPG based on the Doom Engine and dating back to 1996. The developers for this new remake had the wits and sensibility to use the Chocolate Doom engine as the main code base, which is, of course 100% licensed under the GPL. With so many top quality Free Software engine remakes around here, it really astounds me why so few developers choose to use them on modern HD remakes and ports (I'm looking at YOU, upcoming remake of Heroes of Might and Magic III not using VCMI).
As mentioned, so far the game is available exclusively on Steam, but I'm pretty sure a DRM-free GOG version will eventually follow, as it usually does.
Code License: GPLv2 + LGPL
Assets License: Proprietary