With the caveat that I haven't yet had to deal with them personally:
Missile Frigates are the best common counter for them just in terms of damage. They out-range the gravity effect and are only slightly slower than the Gravity Drains themselves (24 speed vs 22 speed). It takes 7.3 Mark I Missile Frigates to kill a Mark I Gravity Drain in a single volley. You get one volley every 10 seconds. Multiply the required Missile Frigates by the Mark of the Gravity Drain. Count higher mark Missile Frigates as multiple Mark Is.
Example, if you have 6 Mark IV Gravity Drains to kill, that requires 30 (rounded up from 29.2) marks worth of Frigates each, so 180 total marks to kill all six instantly. Just a cap of Mark II Frigates would be enough. A cap of Mark Is would kill them in 10 seconds (two volleys).
As for specific tactics, consider Transports to get your Frigates into position. In particular, select your Frigates and click on the Gravity Drains which will set the Frigates' preferred target. Load up the Frigates and fly them into position. If you unload them in range they should target the Gravity Drains first over other targets which prevents them from wasting their first shot on something else. Don't unload them too early because while they prefer Gravity Drains, if other targets are available they may first at elsewhere making you wait 10 seconds for them to reload.
