After trying many, my favourite is Git Extensions [0] (despite the name it's also a stand alone application) for Windows and Linux [1]

[0] https://github.com/gitextensions/gitextensions

[1] https://github.com/gitextensions/gitextensions/wiki/Git-Exte... (2.49, although I just tried 2.50.01 and it seems to work too)

edit: Linux version is a little bit glitchy with UI texts, when pushing to a repository that have commits ahead, the two invisible options are "pull with rebase" and "pull with merge" respectively.