Regarding the issue with mailing lists for open source projects this isn't just an issue regarding Gmail. DKIM and DMARC cause no end of problems for mailing lists and it effects mail delivery to all major providers.
Mailman, which most open source projects use for mailing lists, have developed work arounds to address some of the issues. Unfortunately my experience is that many projects run older versions that don't have these work arounds or if running newer versions they have not been enabled. Most likely because no one has revisited the configuration since initial deployment on an older version. After all they didn't start the project to spend their time being mailing list admins.
Mailman, which most open source projects use for mailing lists, have developed work arounds to address some of the issues. Unfortunately my experience is that many projects run older versions that don't have these work arounds or if running newer versions they have not been enabled. Most likely because no one has revisited the configuration since initial deployment on an older version. After all they didn't start the project to spend their time being mailing list admins.