The basic, default Git for Windows client does include a GUI, however this is not the case on other platforms. See Git for Windows for further info for Windows and a link to their GitHub repository.
Naturally an Integrated Development Environemt, or IDE should support git out of the box. I have been using Visual Studio Code for file editing and as a Git client and I have found it actually works very well. From what I can tell, it does everything you need it to, at least for regular development work with git. For Java development I mostly use IntelliJ IDEA: The Java IDE for Professional Developers by JetBrains and again this has everything you need for git based software development. There are other IDEs available and they should also give you the necessary git support.
A favourite edior of mine is UltraEdit and it has a companion product called UltraCompare, which on Windows supports file comparison in git, which is very handy. Hopefully this functionality will make it to the Mac version before too long.
A product I need to try out is Fork - a fast and friendly git client for Mac and Windows, mostly because it was recommended by Scott Hanselman on Twitter but also because many people seem to like it and it is free, so this is definitely something to try out. It is not an IDE but rather a git client.
I have used SourceTree | Free Git and Hg Client for Mac and Windows a little, it seems to need an Atlassian account to install but after that was fine. In my experience it does not play well behind a proxy server.
I have looked at Git Cola: The highly caffeinated Git GUI but it is more difficult to install on Windows as it needs Git for Windows, Python and PyQt. I got as far as needing qmake to install PyQt and gave up!
Another popular tool is Git GUI Client for Windows Mac and Linux | Axosoft GitKraken but it is only free for non-commercial use on Windows, Mac & Linux. This is the same as Git Client SmartGit which looks worth trying but is only free for non-commercial use.
For a very basic "diff with Git" then take a look at Meld.
Another interesting looking tools is Sublime Merge - Git, done Sublime however it is not free but does look good. It is made by the same people who make the Sublime Text - A sophisticated text editor for code, markup and prose Editor