gxmessage -- An xmessage substitute for gtk-2.0
http://homepages.ihug.co.nz/~trmusson
To quote from the xmessage manual page, xmessage is a program to "display a message or query in a window." It returns an exit code based on the user's response. gxmessage is a gtk-2.0 based clone of xmessage.