GameMaker Studio 2 does not come with a built in diff tool to compare changed and old files in Git. Luckily it is pretty easy to add. Personally I recommend to use KDiff, but there are many different diff tools out there that you can use.

First you will need to download and install KDiff on your computer. Next open up GameMaker2 and go to File -> Preferences and select Plugins -> Source Control.

Now in the Merge section you will add the Tool location to where you installed KDiff. In my case it was C:\\Program Files\\KDiff3\\kdiff3.exe.

For the Tool options you need to add this 

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"${scm_theirs}" "${scm_mine}" -o "${scm_merged}"

For the Diff section:

The Tool location would be the same as the previous location (unless you are using a different tool for the Diff): C:\\Program Files\\KDiff3\\kdiff3.exe

Tool Option:

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"${scm_base}" "${scm_theirs}"

The Tool option is the command KDiff should run when trying to Merge or Diff.

Now when you double click a change in the Commit box it will show up the Diff.

In the Diff you can see what was added and removed in this file for this change.