As a Tyranid player and a Space Marine player, I can obviously see the benefits of the grav guns. Wounding on the strength of the armour, then ignoring that armour, they are amazing weapons for dealing with monstrous creatures. In fact, when I do field my Carcharodons, I live by the motto, “When in doubt, go with the grav.” Yes, I add melta’s into the tac squads, but the Sternguard and Bikes I field usually have grav guns added.
The three main units that can take them are Bikes, Centurions, and Devastators. Why these three? Well, both the Centurions and the bikes can fire the grav weapons at full distance and with the full amount of shots even after they moved. On the Centurions, this means they have five shots each with a range of 24 inches. This also means they have a thirty-inch effective range once you count in their movement. With the bikes, you get a mobile unit that can get six shots at 18 inches, meaning they also have an effective range of 30 inches. The Devastators, while not as mobile as the bikes, can be deployed in a drop pod and wreck a MC on turn one (if deployed right).
Now, like I said I am also a Tyranid player, so I see the other end of the grav guns more often than not. And that 30 Inch range, plus being able to re-roll wounds (in the case of centurions) scares the crap out of me. I have seen Grav Centurions take out the Swarmlord and his guard in the first turn. Some of that had to do with a mistake in deployment, but still, that was 11 wounds, first blood, and slay the warlord giving up in the first turn.
There are counters to the Cents (and other grav guns). Standard troops would be wounded on 6’s, much lower than bolters. Zoanthropes can ignore their armour with a warp lance and cause instant death by doubling their toughness. aThe Zoanthropes are wounded on 5’s, and get a invulnerable saves. Still, even with those counters, Grav guns, Grav-Centurions specifically, give Tyranid players a hell of a rough time.
Am I actually complaining about them? A little. But this was my vent. Now I just have to find a strategy that deals with them. Do I think they are OP? Against MC’s and Terminators, yes. Against anything else? Not so much. They do what they are supposed to do very well. Non-marine infantry, they will struggle against. Of course, the rest of the army is there to deal with those non-high armour save units.