Are Gamespot and Comicvin related sites?

Hi i've been using gamespot for quite some time, when i tried to register in comicvine it suddenly logged me in with the same username that i have on gamespot - is it the same site or something

No

They're connected and owned by the same parent company CBS Interactive. From what I hear they often share content and writers.

Comicvine and Gamespot merged this year. More indept story here with links.