The gar scales are really tough, and way sharp. They cut lines on a regular basis.
Nature has come up with some amazing armor in the case of the gar.

Like Casser said, use tin snips. Sharp knife or not, you are better off with the opposing leverage of a scissors type tool instead of a regular blade.

Here is a link tosome Gar cleaning info and recipies put togehter by the Illinois State Bowfishers.
http://www.chitown-angler.com/1fish/viewtopic.php?t=10673
Sorry for the link to another forum, but I didn't know how to copy the info and make it lookgood. You may recognize somebody in the pictures.