In a CAD, you can take Tiggy as an HQ, an empty Drop Pod as a Fast Attack, and Grav-Cents as Heavy Support.
The idea behind Tiggy is that his Warlord trait makes Hurricane Bolters extra awesome, plus as the best psyker in the SM Dex [arguably his abilities make him a top 5 choice anywhere] he's very likely to snag good powers from a given field... like Div. or Telepathy.
If you like White Scars, and take Khan as your Warlord, you can put a couple Grav Cannons into Devastator unit's Rhino, scout it forwards in the pre-game, and have a pretty good field of fire starting on your 1st turn.
There's very little reason, outside of cost, to not take a Grav-Cannon on a Tactical squad. It's more expensive than an otherwise TAC Plasmagun choice, but you get what you pay for. There are few situations where a GC doesn't outperform a Plasmagun.
I personally don't use Grav-Cents, but I tend to use a lot of plasma and Powerfist-like weapons. People will probably tell you it's impossible to catch a WK, but it's not. You have to grind it down a little, tie it up with a fast-but-unlikely-to-do-much unit, and then hit it with Hammernators / CM on Bike. It's not nearly as simple as Grav-Cents, but it has it's rewards.
It can be a tough challenge to deal with, especially multiples! But it can be done.