Same thing happened to me. I looked for XQuartz after the ML upgrade on my hard drive and found that it had not been uninstalled. So I ran Gimp and it requested XQuartz be installed and opened the download page. I downloaded and installed it, then I attempted to open Gimp again. The program then requested the location of XQuartz and I identified it under Utilities and Gimp started running.

Go ahead and open a bug report. I don't that this is expected behavior.