What is the correct way to secure a gas standalone range?
There is an inexpensive commercial product called an anti-tip bracket.



It is screwed directly to the floor and the foot of the stove slides into the slot.

It's primarily designed to go on the rear foot, so that a child climbing up the front of the stove won't pull it down on top of him/her self.  But you slide the stove into the rear bracket, and then attach one in place at the front for extra security.  The problem of course, is that you'll have to remove the front bracket to clean behind the stove.