Another thought is this - if you have ruins around, park your Riptides in them. Obviously, the nova-charged shield is better, but the 4+ cover save from a ruin can be used against grav cannons.
Another possibility - though this is a bit of a one-trick death star too - would be this anti-cent unit I cooked up:
6 Pathfinders, 2 shas'la with rail rifles, shas'ui with rail rifle, 3 ordinary shas'la, two gun drones.
Darkstrider, two gun drones
Shadowsun, command-link drone, 1 MV52 shield drone
You get a unit with seven 5+ saves, six 4+ and one 3+ - so the grav cannons wound on 5s - never worse than a 4+ cover save, at least one 3++, re-roll 1s, and 5-8 AP1 shots, two of which are at BS5. Also, the unit will be T4 versus your shooting, so the fusion blasters will insta-gib the centurions, and the rail rifles can gib the special character unless he has EW.
Of course, there's no guarantee the cents will actually shoot at it, so maybe not.