> To use the compiled binaries, you must own the game. > This repository and its contents are licensed under the GPL v3 license, with additional terms applied. Please see LICENSE.md for details. It does not sound open-source to me...
I mean, it sounds like the Red Alert code is available under GPLv3 (plus some additional "we want to be very clear, this doesn't cover the trademark" terms tacked on), but you can't build it without DX5, which gets compiled into the binary, thereby making the binary non-free. Someone could port it from DX5, GCL, and HMI and produce something that builds as GPL. The other part is it doesn't include the game assets or…
They first statement about DX:
> or write new replacements (or remove the code using them entirely) for the following libraries;
But, they say that binary is proprietary regardless, no conditions. So it is very difficult to say.
If the code compiles without assets (no mention about them, it sounds like it should compile), then the resulting binary should be free to use. Missing runtime assets are different problem, and separate from the binary usage permissions.