Can I buy a samsung gravity and put my old sim card into it?

Hi yes you can use your old sim card in samsung gravity phone only if it is unlocked.Else if the phone is locked to some other network provider then until you unlock it you can't freely use it with any GSM sim .You can easily unlock it using sites like ProUnlocking.com here they are providing unlock code for all mobile models at reliable cost .After unlocking the phone you can freely use it with any GSM sim .