Example:
gtf 1280 1024 60
Save the resulting output as an example:
Section "Monitor" Identifier "Monitor0" Modeline "1280x1024_60.00" 108.88 1280 1360 1496 1712 1024 1025 1028 1060 -HSync +Vsync EndSection Section "Device" Identifier "Card0" EndSection Section "Screen" Identifier "Screen0" Device "Card0" Monitor "Monitor0" SubSection "Display" Modes "1280x1024_60.00" EndSubSection EndSection