Uff, Grav/Storm, welcome to GODMODE. Grav/Storm is my other main. I have tried to find a combo that works better, but haven't yet. Well, aside from the old Ill/Rad, but that's cheating, so whatever. Some tips:
1) Slot everything for damage that takes damage. Slot Snowstorm for slow, slot Grav Dist Field for Hold, Hurricane for -ToHit.
2) I do recommend slotting with yellow attuned IOs, unless you can afford reds, of course. Note that I am an IO addict, so I might be biased. I love Attuned because they level up with you, so once they are slotted, there is no need to swap them out until 50 when you start Purpling. Saves so much time at the store.
3) Now for some tactics. Hurricane is an end-hog and an annoyance, but that -ToHit and -Range is so sweet. Generally only use it in 2 situations. 1) You get overwhelmed and need to run. Turn it on and blow (literally) through all the opponents to get clear. or 2) You are up against a hard target, like a Boss, AV, EB or something. Those won't be tossed and reducing their hitting you helps a lot.
4) Crushing Field, spam it for AOE damage. Also good as mentioned above to keep things in Freezing Rain. With Freezing Rain and CF spam, you can wipe out large groups of minions pretty quickly.
5) Wormhole, literally my favorite power in the game. If you team a lot, drop in one of those KB>KD enhancements and suddenly it goes from a conditional/annoying power, to one of the most useful powers in the game. The thing I did not realize till I did that is that Wormhole actually deposits your opponents in a tight pile. This is great for stacking dispersed enemies tightly on a tank. I have seen people tank with Wormhole as their opener. BOOM suddenly you have a bunch of delirious baddies where you want them. I do slot WH for stun, but at lower levels, make sure you have plent of ACC in it. Solo, I like to drop Freezing Rain, then WH them into it, and lock them with CF. Amazing opener
6) Slot the heck out of Singularity. It is embarrassing that it is a better controller than you are, but it is okay since it's on your side. I 6-slot it as soon as I can. It will save your life many times over.
7) Lightning storm is another life-saver and a good source of damage.
8 ) When you get Tornado, slot it with one of those KD>KB enhancements and suddenly you have yourself an invulnerable tank. I learned this trick from other Stormie. You open by putting Tornie out in the middle of the spawn mob, it can't be killed, and the alpha damage on you is mitigated because they are all falling down. Then slot it for Def Debuff, and you have a nice boss grinder.
9) Once you get all your Stormie powers, you can lean on them for DPS. The ones that do damage are quite good at it.
10) Also, your pet, Singularity, and pseudo-pets, Tornado and Lightning Storm, are actually quite smart. There have been many a times when I have been working on one enemy only to turn around and see that they all have chased down and cleaned up the rest. It's weird, but lean on them.
11) Try to get out of the habit of using Gravity Distortion Field on the regular. I know, some people may say that is heresy, but really, you rarely need it. But, when you do need it, you NEEEEEED it. Just leave there, unused, until the sheet hits the fan, then throw it out there to manage whatever mess happened. Like, if there is a big ambush, drop GDF on it, and boom, handled.
12) In teams, you are going to want to rush ahead and tank because everyone else is too slow and mouthbreathing (This urge gets especially bad when you get your Epic Pool). Try not to do that. You are best as rear guard, cleanup, and disaster crew. Do NOT put Lightning Storm in the middle of the spawn. Put it towards the rear or any escape route, that way it can go ahead and zap anybody who decides to run. Singularity sits over your shoulder and guards your back, and is also good for nailing runners that you miss. I have seen Singularity single-handedly break up massive ambushes and keep the whole thing busy until the rest of the team realizes that there was an ambush and comes to put it down.
13) Another reason to stand back a bit is that if things get tough, you can turn on Hurricane and give cover to the other squishies. In addition,your Hurricane can stop any runners that try to get past.
Well, that should get you going. Welcome to the mayhem!