Firstly, a unit of 3 grav centurions puts out 15 grav shots because they are armed with grav cannons rather than grav guns. Given this, they can reliably demolish most units in one turn of shooting. That said, you're opponent is paying (guesstimating) about 350-400pts for that unit, so if you can destroy it, then you will be putting a serious dent in your opponent's army.
Now let's discuss the ways you can destroy such a unit. First, if he tells you that he's bringing 4 centurions and two characters in a drop pod, politely inform him that it is not a Tardis and that his centurions are Very Bulky, and thus he cannot fit all of that into one pod. If he instead brings a 3-man grav centurion unit with Shen, here's what you're facing: 15 grav shots that, on average, will get 10 hits with 9 wounds on average (8.9 was the exact number). With a riptide's 5++, he will only pass 3 of those, which leaves him as a very dead riptide. You, however, can turn this to your advantage. Assuming average rolls, taking shield drones would allow your riptide to escape with one wound and no drones from such an onslaught, but you'd be taking a morale test (which you never want to do with a riptide). Or, when you castle your forces, you could put your riptide out front to screen less assuming units and bait your opponent into killing it first. Then that one unit has an entire army to deal with after killing only about half their points value.
For dealing with this unit, I would recommend Hammerheads. Take 3 and support them with markerlights, specifically marker drones (either on their own or in a crisis team) led by a DC commander. 3 S10 shots will force your opponent to do one of two things, either bounce the shots onto the centurions off of Shen, or sacrifice Shen to keep the centurions alive. He's not getting his FnP either way, and each shot that successfully wounds dismantles his unit that much further.
Now, for castling, kroot are a great bubble wrap unit. A unit of 20 with a shaper can provide excellent coverage, and at the maximum 2" coherency, more so. The shaper also provides solid leadership for a small points investment. Two units can cheaply push your opponent's deep-strikers far back, giving you some breathing room for your essential units. They're also really inexpensive, too. A squad of 20 Kroot with sniper ammo and a shaper are 160 blacksun filters. Additionally, if you give them sniper ammo and are lucky enough to force your opponent to sacrifice Shen, you can light up that unit with sniper shots galore.
If you want to talk further, feel free to hit me up with a PM. I have an appointment to keep this evening but I'll get back to you when I can.
Unity is a rock against the tides of conflict.