It's a kind of qualifier at the end of a sentence like "right?", as in "I'll have the blue one, right?"
I was told it comes from "geil" which can mean sexy or hot or wicked (bit of a saucy connotation!) but has been tempered down to just mean a qualifier to a sentence.
It even comes in its polite format "gellen Sie?" which, to a non-native speaker, seems a bit inappropriate...