When I recently reserved ODST at Gamestop, I forgot to ask if I will get Johnson, will I or is it only available for Preorders
I'm pretty sure that reserved counts as preorder, but if so then you should have probably already gotten a case with the code. If I were you I'd ask your local gamestop to find out.
Reserving and pre-ordering are the same thing. If you pre-ordered/reserved the game from gamestop, then yes, you will be allowed to play as Johnson in FireFight mode.
I was concerned because when I went to pre-order mine I didn't get anything. I was lead to believe you'd get something in the store, when you pre-ordered. I went and asked them a day later and they said they didn't have anything to give and that they thought you'd get it in an e-mail or when you pick it up on the 22nd. I'm a just a bit concerned.