I have plenty of experience with the Bianchi X-15, but not the Glock.
I had a Medium X-15 for years and used it with a Colt Government, Colt Commander, S&W 3¾" & 4" N-frames and 4" Charter Arms .44 sp Bulldog.
The X-15 is very adaptable and forgiving as to fit.
What would concern me is, since the X-15 has a U shaped spring around the opening to retain the weapon, it might draw the slide back somwhat on the Glock. You have to insert the gun and then push down.
I carried my Colts Cocked & Locked so slide movement was not a problem. But if I put it in hammer down it would push the slide back to half cock.