what do you feel isnt working for you? Would recommend getting in touch with Kinetic, as his grav/ff is astounding, but I'd be interested to know where you're looking to improve, will try and give you my 2 cents worth 
What's stumping me is planning an entire build in general! :P I haven't actually created the toon yet, I'm setting up a build in advance for it but powers, slots and all that jazz has got me too confuzzled to get ahead with it.
Right, well, my experience comes from a bubble defender, so I'm automatically inclined to focus on the bubbles. Friend runs a Grav/Kin so i'm ok with the grav set, and if you want some first hand hints, /t Kinetic 'ere he's ingame.
First, do you want to focus on holding/phasing/mob placement, or would you rather keep your team protected under your bubbles, and hold when the buff cycle is finished?
Propel being your only realy dmg output should be 6-slotted, i feel, as should your single target and mass hold. Phasing is up to you, most don't take it, but in the right situation, can turn a dire looking situation into your own hands.
On bubbles, i skipped a lot of the pool in place of the leadership pool, and tend to recommend skipping force bolt (the KB attack) and the repulsion field & bomb. The cage, 2 buffs and two PBAoE bubbles (including force bubble...sooooo amazing in the right situation) are enough to make the most of the bubble set, especially as a troller /2ry.
Wormhole i'm unsure of as to slots, but another situational power that, when used correctly, is very effective, and excellent when teaming with any sets that use a lot of AoEs and require the mobs to be close together (this is also aided with Force Bubble's ability to repel foes into and keep them in the corner)
/rambling rant...any specific questions that aren't anwered in that stupid ramble (e.g, would you like me to build something on mids for you to look at?) then just ask here or pm me ingame.
Stick with the set though. Is an amazing team set that I personally swoon over
Hiya Grav/FF it's fun - tough one to play I found I've got one to level 50 and it was a long hog - i'd say respec is our friend.
At the low levels having propel & the ST hold coupled with the bubbles for allies means that you can play well within a team and still have enough ST dmaage to get through the lower levels.
As mentioned above skipping the Repel style powers from FF is an option (one I went for too) although forcebolt is a handy tool to keep a mob tied up in a sticky situation - think you have to have it but an Acc SO or two would suffice for slotting on it.
From grav for leveling I'd skip the Immob powers and D-shift - reasons:
1) I'm not too keen on the imob powers at low levels I don't think you have the powers to hold the aggro and it gives alot of aggro - the ST immob at low level can be replaced by alot of better powers and taken later in the build.
2) D-shift - well although I do love this power it really doesn't hold well in alot of teams (especially PuGs) it's a real panic hit button to save the team but regular use will lead you to becoming more porular - with the other AoE control powers I'd advise against this until higher level.
for pool powers medicine pool is great not only for the self heal in PFF at those sticky moments but the bit of healing you can offer, stamina I'd say is a big must too as running the PbAoE bubbles as well as spamming your attacks/holds will create an end problem unless carfeul slotting is adhered to.
Last couple of points - Wormhole - YES - great power, very long stun when slotted - careful on the KB it does as can put mobs out of AoE. It's recharge is good and the conrtol factor fantastic - Sing (the pet) big Yes again - he'll help out with the control while you are applying bubbles if mid fight or trying to cover yourself in PFF if the heat is too high.
Hope the ramblings have given you some indication - it's a tough build to solo with (I found at least) but it is a very rewarding one to play and great in teams. If you want any other pointers give me a shout and am happy to rant on some more
Id have to say Force Bolt is one of the stand out powers of the set, IMO. Dont dare skip it.
Now it can be used clumsily, just be knocking random enemies out of the melee.
On the other hand, it can be used on the boss, perma knockbacking him/her against a wall (or even better, a corner).
With its built in accuracy bonus, all you need is one slot (acc or maybe an acc/end KB IO), you have a power that can take a boss out of fight from the word dot with one slot.
This is not skippable! 
Well I tried to plan my own Grav/FF troller build, but in all honesty I'm just not experienced with controllers or the powers enough to really make it work, so if anyone has any advice or even a build I could use, really appreciate it.