Did you use the special $15 tool that people advised on? Or did you use a screw driver and a hammer. Anyway, it looks like the special tool will no longer be an option for removal because the ring looks quite destroyed.
In any case, it is a few full turns before it comes off. It look like you are going to have to cut it very carefully. It will pop off like a spring. But don't cut the underlying plastic frame! But before you do so, get a new ring first as you do not want to have an open gas tank sitting there while you get a new ring at BMW tomorrow. Oh and get the tool to close/torque it properly. Get this one (which I got):