Is it possible to view git diffs using a GUI side-by-side tool on Mac?

You could use opendiff. It is a command line tool which opens the GUI of FileMerge.

You could instruct Git to use it automatically for git-mergetool with:

git config --global merge.tool opendiff

If you want it for git-difftool as well:

git config --global diff.tool opendiff

And you could also disable the prompting for every file with:

git config --global difftool.prompt false

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For more details type: git help config and search with / for the different options.

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P.S. If you don't have opendiff installed you could install it together with the Developer Tools from XCode: https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/opendiff.1.html

UPDATE: In recent versions of XCode, FileMerge is now bundled with XCode. You cannot install FileMerge as a standalone program. opendiff is still in its command-line utilities which are standalone.

P4Merge from Perforce is pretty good and the standalone P4Merge application is free. You can get it at http://www.perforce.com/product/components/perforce-visual-merge-and-diff-tools

Also, if you are using SublimeText than http://www.sublimerge.com/ is also a good tool.

Even if you aren't using Python, having PyCharm installed is worth it for its great Git integration. It's a good general-purpose editor and has a good visual diff+merge tool. The merge tool includes a triple-pane interface with intuitive code block selection. The community edition is free and works on my installation of MacOS High Sierra.

I use BBEdit for this purpose. I think its diff display works very well. I have it set as my git external diff tool.