I don't have thin hair, but my hair is easily weighed down because it has a weak curl pattern. I was using the Garnier Fructise Pure Clean Foritfying conditioner too, and omg did it weigh be down. I think you are right to step away from that product. It is also incredibly emollient-heavy, which I think means it could overcondition your hair easily, which could also leave it looking a little limp. I have not tried Tresemme Naturals yet, but I want to! (I just decided/discovered two days ago that the GF condish was weighing me down, haven't had a chance to buy anything else yet). In the meantime, I have been using Suave Naturals Everlasting Sunshine as a rinse-out and a leave-in. It is very light, and it gave me much more volume and curl than the GF pure clean. I am not sure it is moisturizing enough for me, but if you have any Suave Naturals already you could give it a shot.
I also use the GF pure clean gel, and I agree that it too is a little heavy. How much have you been using? I have dense hair that is just shy of shoulder-length, and I discovered that I need very, very little of that gel in order to get good hold. I use about a nickel size for each side of my head, and then a dime for the back of my head. You could try scaling back the amount and see if that helps.
As for additional conditioner recommendations, Iím a newbie so I'm not much help. I just discovered this thread though: The Conditioner Chronicles
It's old (2006), so definitely check ingredients before you make any purchases, as the products could have changed. BUT this is such a great list of a ton of conditioners out there, and you can search through for conditioners that are more lightweight.
Lastly, a mousse might be a good idea to try, at the very least! I experimented with one, and it did really boost the overall volume of my hair. It was the Pantene Curly Hair Series Curl Defining Mousse. (link: Pantene Curly Hair Series Curl Defining Mousse - NaturallyCurly.com
) It really, really gave me a lot of volume and some pretty decent curl definition with little frizz. My only issue is it has a polyquat in it, and that caused buildup for me. I didn't find the mousse to be drying, though, so that's good.