I finally found a number of posts from internet search that id'ed it.
And the owner who lent me the gun (on the road in Utah) showed me the box of ammo he uses.
Which is the same as .45 acp.
I do not believe one can use gap ammo in an acp gun, nor vice versa.
Another friend and I went out to the Wayne county Utah county range. Closed for leo day, but they directed us over to the end of the rifle range where we tried the Glock 21. It's a nice gun, and a good shooter, and accurate and all, but I'm not a Tupperware person.
However the range is in some of the most beautiful scenery in all of the USA.