do you mean for the gzdoom build itself? im 90% sure thats the same as normal gzdoom with like one minor tweak to the window name?? as for the game assets and code itself, i dont think the same license applies to stuff like mods and standalones made for gzdoom, since they can technically run under other source ports. also it should be noted that all wad and pk3s are technically open source (paying to get the file i guess might not count), since nothing aside from the ACS scripts are compiled.