Gape is a product of AIR (or gas, for that matter) being trapped inside large intestine and rectum. Plain and simple.
The act of having something simply inserted into the anus will not make someone gape. It's kinda like a queef; air is forced in as the object is thrust in and out.
The air has to come out somehow, and the position the body is in will determine if it will allow the sphincter to gape.
If you notice in porn, there are only two ways the girls will produce a real wide open gape: on all fours with their ass pointed up in the air, or on their backs, legs up, with their asshole pointed upwards toward the ceiling. This is because the air that is pushed in quickly takes the path of least resistance when the sphincter is positioned upwards.
This may be weird, but you can duplicate this exact thing in the morning when you wake up without ever putting anything in your ass. Simply go on all fours, ass pointed up in the air. Relax, and within a minute you will probably feel gas moving up out of your bowels towards the exit. If you gently "let go" of the gas - not forcefully like a fart - your ass will gape. Most likely multiple times. For some reason the sphincters just opens up. Sounds rificulous but try it and you'll see what i mean.