You are reading it wrong. "Gitlab does not run on Windows" means that you should deploy this on a linux _server_. You can still use windows on your local machine, and push your changes to a linux box. It's true that you will have to have another machine, or a virtual box to use gitlab, but I believe that's how small/medium companies work.
> There exist plenty of genuine windows devs out there that use git, or devs that switch between OS'es for different tasks (myself included, I'd boot up windows to work on graphics/games)
Yes, but you probably shouldn't be running a code hosting platform on one of your developer workstations.
I'm pretty sure it would work from a client point of view (it's still git after all), just that they won't support you running their web app on windows.