What do you put on a ganglion cyst for treatment? Ice?

Nothing. Splinting the joint and the use of anti-inflamatories has the best chance of getting the ganglion to receed in size and posibly reduce any pain.
No ice. If in fact you have a ganglion cyst, this is a fluid filled sac communicating with the joint. Ice helps with inflammatory issues for the most part. A ganglion cyst is not an inflammatory condition. Once the diagnosis is confirmed it can be left alone, aspirated, or excised. Best of luck.
Not much. Unfortunately there are no topical treatments for ganglions that I have found effective. You can try an aspiration where a needle is inserted into the ganglion to remove the fluid. If it continues to cause symptoms you can consider a procedure to remove the ganglion.