When I open it, I get an error saying this:
runtime error: OpenGL error: GL_INVALID_ENUM - An unacceptable value is specified for an enumerated argument.
The error is so long that it did not all fit on 1024x768 so I have a screenshot for both the top and bottom.
Top part of error dialog:
http://img2.freeimagehosting.net/uploads/0decacf117.png
Bottom:
http://img2.freeimagehosting.net/uploads/c7f1c83d3f.png
It asks me if I want to break into the debugger. If I click Yes then GtkRadiant closes. If I click no then the same error pops up again, and if I click no again then the program will stay open but when I close it Ubuntu crashes to the terminal and then goes to the login screen.