I didn't mind this as a way to back up my stuff on the off chance I ever lost anything, but once it started to get it a bit full I thought I'd remove some stuff that I didn't need to keep forever. Turns out, you delete it in Google photos, you delete it off your phone entirely, I lost hundreds of photos because I assumed it was separate to my gallery but apparently not. And I cannot find a way to stop it, so now my Google account is clogged with things I don't need on there.