I think they mean the commit message editor, which git will use to open a temp file to save the message to if you don't specify a message in-line with the "-m"

I think they mean the commit message editor, which git will use to open a temp file to save the message to if you don't specify a message in-line with the "-m" flag when committing, including when a merge commit is initiated by a "pull". This happens on the CLI, it's just usually (though doesn't have to be!) a command line editor that it opens. I think vim's a common default. AFAIK whatever's opened does need to block the CLI, so you can't use a command that opens a GUI editor then returns immediately or git will interpret that as your having closed the file without saving, but otherwise any editor should work, CLI or GUI, and can be assigned in your git config.