Launch the script gp2xsleep_and_menu.gpe.
If you want to get your gp2x to sleep press L+R+middle volume.
CPU will go down to 30Mhz, backlight will be disabled and the lcd chip will be turned off.
Battery led will blink every second.
To go back to normal mode, use the same combination again.
The program stays active while the gp2x is running.