Does anyone have this on thier car?

Just bought a used 60 plate car, value is around £17k and just assumed I should get GAP insurance but some people are telling me otherwise. The dealer tried selling it to me but wanted £500 for 3 years, looking online I can get it from ALA (who everyone seem to reccomend) at £114 for 3 years so seems worth it to me.

But is it needed on a used car or is it only really for brand new cars on finance?