In Gerrit, under the download section for a change, there is a section that allows you to download the Patch-File:
Using git apply
results in:
$ git apply 441eb56b.diff.base64
fatal: unrecognized input
What command do I use to apply this patch?
base64 --decode c6a9dcdb.diff.base64 > c6a9dcdb.diff
git apply c6a9dcdb.diff
(Replace c6a9dcdb with whatever abbreviated commit hash Gerrit gave you.)