An expensive alternative is Kaleidoscope app, which looks great but might be overkill for our purposes.
The good news is that XCode provides a GUI tool called FileMerge, which is also
opendiff from the command line. You do have to agree to the Xcode/iOS
license, which requires local admin privileges, to use this tool.
$ which opendiff /usr/bin/opendiff
It appears to provide a very intuitive GUI representation of the differences. You can also configure it to be the default merge tool with Git.
git config --global merge.tool opendiff
If you want to launch FileMerge from the Launcher, you’ll need to open the Applications folder, right-click on XCode, choose to ‘Show Package Contents’, then navigate to Contents/Applications. Inside you’ll see the FileMerge application.
If you right-click on FileMerge, you can right-click, choose to ‘Make Alias’, then move the alias to your Applications folder.