You are on the hook since the judgment was already entered. The statute of limitations only comes in to play before a judgment. The debt is valid until GMAC reports it as settled.
You can either try to work something out with them for a lesser amount (they will be happy to receive just about anything after so long), or you can play the collections game with them and wait for them to go back to court and try to garnish wages or place a lien on property. Since the car was repossessed and surely resold, you can likely get the debt amount significantly reduced. However, you can test them and see how determined they are by simply not responding at all.
Your granddaughter is still responsible too. Get her involved and make her pay up.
Also... check your credit report to see if the judgment is on there. After 10 years it probably should not be. If you suddenly start making payments on the account they might be able to place the debt back on your credit report. If you do try to work something out with them see you can prevent them from doing that. Let them place it on your granddaughter's credit report if she does not cooperate.