gmplayer runs fine if called from an xterm, but it will freeze uwm if called from the menu. The mouse pointer can still be moved, but it doesn't respond to any mouse clicks. A ctrl-alt-backspace is necessary to exit uwm.
Tested on:
Slackware 13.1 32 bit - Nvidia binary drivers
Slackware 13.1 32 bit - VirtualBox
Slackware 13.1 64 bit - VirtualBox