Hi Klaus,
Git is cross platform, so unless you want specifically some Windows GUI front ends, it's good to read up in the core git functionality, as it applies universally on any platform. Here is a good free online book:
https://git-scm.com/book/en/v2Using github and the like - they are just interfaces over core git, and github for example has extensive online help about github specific features.
The UNIX man pages for git are clear and helpful, and even include git tutorials, and summary pages of the 20% of commands you use 80% of the time. [They don't put it like that, but you know what I mean.]
Andrew