No you are just buying a sub-domain from an individual (the person who owns gb.com) - not a real domain.
If you want a UK domain, get a .co.uk. The gb.com ones are rubbish. It's not a real TLD. It's a subdomain. I'm not sure what the SE's think, but .uk would probably be better.
they are waste. all your efforts will be vanished if the domain sold to other party as it will be risk to your subdomains from that domains
Why do you want to buy a TLD like that? It is a simple subdomain just like co.cc etc Do NOT buy such as TLD it is a waste of money! Good luck