It really depends on the GUI. First time users find TortoiseGit very confusing and would basically brute-force their way to getting something checked-in, which means that they might or might not mess up the repository. In a hackathon I participated in a few months ago first time users were able to break the repository (fast forwards, bad merges, etc) on several occasions. They weren't able to get a good overview of what's going on. git-gui and especially SmartGit were much much easier to comprehend and work with (git-gui however being very limited in functionality).
(Moved to be an answer as per the comments to my comment).