First of all, gamban or gamstop? 2 totally different things. If you’re on gamstop and you willing played at the casino, I highly doubt you’ll be paid. At it’s best, you may get your deposit refunded.
Gamban, from what I remember, only stopped you going on those websites, so was nothing to do with self exclusion. Just like a website blocker.
You say you had never heard of the exclusion before, but signing up to these things isnt a small thing to do, and not something you would forget.