XQuartz / Gimp problem

After upgrading to Mountain Lion and installing XQuartz I have the following problem:

Symptoms - double clicking the Gimp application activates XQuartz but Gimp does not start. The icon bounces once or twice and then dies.

I was able to get Gimp to run by customising the XQuartz application menu by inserting a new item for Gimp with the command path as follows: /Applications/Gimp.app/Contents/MacOS/Gimp. When this is selected from the XQuartz menu Gimp launces and behaves normally. But even after this it won't launch as a result of double clicking the icon.

Before the upgrade I was previously using X11 and clicking on the Gimp icon was all that was necessary. My question is whether this is normal behaviour under XQuartz or is something wrong. I would prefer to open Gimp by the double click method as before. I have no other X11 applications in my Applications folder, so I cante test them to determine whether the problem is with Gimp or XQuartz. Xclock and Xlogo work when called from the XQuartz Application menu but not when invoked from Terminal (which they previously did). Invoking Xclock from the Terminal results in the following:

Last login: Fri Jul 27 17:27:58 on ttys000

iMac:~ peter$ Xclock

Error: Can't open display:

iMac:~ peter$

I have also noticed that I have a new folder (Opt) at the root level of my hard drive. I'm pretty sure - but not 100% certain - this followed the installation of XQuartz.

Versions: Gimp 2.8.0p1 and XQuartz 2.7.2.

By the way I am really impressed with Mountain Lion. My machine is a mid 2007 iMac with 3GB of RAM. I was reluctant to install ML as my machine is the oldest one listed in the hardware requirements but I thought at $20.99 I couldn't go wrong. After running it for about a day I am glad I made the change. It seemed sluggish at first but now seems faster than Lion.