this is all caused by "forgetting" your acctual nickname... so to solve, think of your NICKNAME that you really used when u made the gamecenter account.
then go back and sign in, which it should know as for "new nickname" BUT rather than making a NEW one, enter your real and REMEMBERED one, type it in and then done, it will then let you in, however if it says "in use" then try with a cap letter, if it still says it, then its not ur nickname, so think harder untill u remember, this is the way it is, i did the same, but then i was going to make a new one in the end, but my NEW nickname, happend to be the one i really used when i made it ^.^. and so question:
how does it not let you have OTHER peoples nickname and says its in use, but can alow you to have yours back ? = simple, basicly its because your EMAIL nd GAMECENTER nick name, are in a JOINT database, meaning it will let you have yours back because its in your data field, which won't let you have someone elses because it detects your not using their email to login, so says "in use".....
think hard, and your find it 😝
Good luck all
p.s. no matter how much u "insist" on knowing you have the correct nickname, just think, because i did the same, but i was wrong, soo, hope everything goes well.......