I haven't personally used self-exclusion but I'm a bit familiar with how GamStop is supposed to work. The scheme has all casinos and betting sites licensed by the UK Gambling Commission in its network. So when you self-exclude from GamStop you automatically self-exclude from all UKGC gaming and betting platforms. And I think you can't reverse the ban at all unless the period you've chosen passes. Even if it's over, they won't let you know about it and you must contact them to remove the ban.
But I think that it's connected to your IP address or maybe your device, not sure about that. Plus, it doesn't restrict offshore sites. So, it's not effective 100% and I agree with everyone saying that the person must want to stop gambling to be successful. It's so easy to gamble these days, it really takes dedication to stop doing it. But I hope with all the self-restriction tools and support that anyone with a problem could successfully deal with it.
Of course, national self-exclusion schemes like GamStop in United Kingdom don't work for offshore gambling sites.
But any serious, reputed and professional online casino or sportsbook should offer a self-exclusion option and apply a solid Responsible Gambling Policy IMO.
For example on Stake you're able to request for being temporarily or permanently self-excluded whenever you want and the platform will prevent you from using any other account to try to continue to gamble during your self-exclusion period.