What is a better GM gun?

Anaconda.
Or Anaconda ADV if you're feeling lucky with the crates.

If you want a valid review of the three guns you stated:
K2: Great for single-shot. If you have decent aim, you can get HS from across Powerplant. When using Reticle #2, the bullets will stay at the top of the reticle when at full auto.

M4A1: Great for burst firing at midrange. Capable of getting HS easily. Not a very good rush gun due to relatively low damage for an Assault rifle. Full auto is less predictable, tends to go up to top of reticle #2 and then move side to side.

AK74: Not a bad gun. Not a good gun. It doesn't know whether to be an AK47 or an M4A1. Does the same as a K2, but not as well. I don't reccomend this gun.

For someone else's opinion; look up FinalShot Review <Name of Gun here>

awm + deagle/anaconda.
Each GM map gives you a nice change to take out a few ghosts with an awm at the start of each round (most ghosts don't know it on lab/lostrelics) Later in game you still have some nice places to position yourself with the awm.
Your pistol is for moving around, 2 bullets to the chest is enough to take a ghost down at close range. Ghosts need to be close, so you'll be able to use your pistol at its full potential.

when you see ghosts, the first thing to appear in your mind is kill 'em kill 'em
MP5 does the job perfectly,
SMGs and Shottys works perfectly
Rifles are good too
Snipers are not recommend

use a silencer weapon

berreta is good for GM

thomson ...is best weapon on GM ..cuz u can move fast and have sufficient bullets