Was doing a bit of research for my Garg art when I found this. It didn't make sense to me as I haven't seen a Garg actually make long jumps.
http://www.shikadi.net/keenwiki/Garg
"Gargs can be killed with only one shot, but their speed and ability to make long jumps make them a dangerous enemy. "
Don't they just stay on the ground? Or is this assuming what might happen if they ran off a cliff?
No; they only jump if they charge off a ledge. There aren't many situations in Keen 1 where this can happen, however, it is frequently seen in mods and levelpacks.
This effect can be produced in the "Emerald City" (the one blocking the way to the teleporter). There is a Garg hidden in a yellow pipe-like thing near the bottom of the level. And there is plenty of empty space for him to jump.