I used the Gerrit Web UI to create a branch in a project. Now I want to delete that branch, but do not see any "delete" button next to the branch in the Web UI. Can I delete the branch using the Git client? Or, will attempting to mix Gerrit and Git in branch creation and deletion cause problems for the repo?
If you don't see the "DELETE" button in the Web UI, probably you don't have "Delete Reference" permission. See more info in the Gerrit documentation here.
Answering your question: yes, you can delete a branch using the Git client but you also need to have permission on Gerrit and then execute:
git push origin :<branch>