Garage Disconnect question

I have a detached garage with a single 20A line running underground to the garage, my question is do I need to have a disconnect inside the garage since it's detached? And does it have to be an actual purpose-built disconnect switch or would a double-pole 20A switch do just as well? (cut off both hot and neutral) Already plan to wire in a GFCI as first device to protect down-stream, could the GFCI act as a disconnect?

Thanks in advance!