You will be shooting air

Just like visible photons, gamma interacts with air. Air absorbs gamma. So — when extrapolating into the MeV range from the table on that page — for every 100 meters away your target is, you will be putting half of your remaining energy into the air. At 1000 meters, your gamma ray is 1/1000 of what it was at the gun.

This of course means that you heat the air immensely, which will distort it and obscure the target.

Absorption, obscuring and blooming are known problems with directed energy weapons.

In short: this will mostly just punch holes in the air.