I would point out that GitHub Desktop at least, is available on both Windows and Mac, and I was told yesterday is in beta for Linux. ;) I think it's quite common for version control tools to be cross-platform since they are often used to develop cross-platform software.

I would say the people making active participation in your mailing list or issue tracker, and especially those making patches to your actual software, likely represent the most active/invested members of your community, and definitely the best people to solicit opinions from. :)