But first you need to use the force to shove it in the zombie's neck, then pull it back out with just as much force, then sheath it.
Gun = Pull trigger, keep running. It's not like you can't shoot a gun up close.
I'm pretty sure it was you who said that one would need a high-power gun to do anything whatsoever to a zombie. If not, that's still a good point to make, because most high-powered guns are 2-handed.
So you're telling me you would turn around, losing your speed, square up, and shoot the zombie? Because a glock wouldn't do stuff. And what if you're out of ammo?
For that reason, a melee is entirely necessary as a back-up.