The source code for Doom is GPL, but the .wad files and assets that Doom needs to run are not GPL and are not included in the source code. It's the text files containing application code that are open source. And GPL specifically says that the software has no "FITNESS FOR A PARTICULAR PURPOSE", so it doesn't matter if a piece of GPL-licensed software is not usable.
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The source code for Doom is GPL, but the .wad files and assets that Doom needs to run are not GPL and are not included in the source code. It's the text files containing application code that are open source. And GPL specifically says that the software has no "FITNESS FOR A PARTICULAR PURPOSE", so it doesn't matter if a piece of GPL-licensed software is not usable.