some good things to have:
tyranitar- he makes a great stealth rocker, while setting up sand for sand force landorus to wreak havoc with its quakeedge coverage. can abuse it as well
nidoking/queen- one provides sheer power(geddit?) other does the same but the added benefit of t-spikes utility
a good, strong fighting type. takes out blisseys/chanseys that would normally wall half the gravity abusers out there.
starmie- spins and great, high powered coverage moves. can also set up gravity.
blissey - takes any and all special hits the opponent might use to abuse gravity, i.e. thunders, fire blasts, the odd blizzard, hydro pump, etc.
those are just a few pokemon who can help fill certain niches in gravity teams. generally, i classify them into 2 types: offensive and stall based, like hail.
stall based key points- gravity and spikes is a great combo, being able to get 3 layers down benefits greatly, taking off 25% each time. slowly wear them down. typical stall.
offensive - these take a more direct approach. generally, you pack a couple sweepers(who make a great offensive core) with a couple high power, low accuracy moves and let them run rampant. keep in mind, you'll still want at least a couple 90%< moves to have a good team. the good players learn fast, and wont keep letting you set up gravity.