Cloaks can be tricky, not bad for a first attempt but a little too bulky and thick as you say but easy to redo
What I do is mix the GS with a little less yellow than blue and give it 20-30 mins to cure, then use either a piece of plastic (ziploc bag) or teflon sheet (get these on ebay easily) and roll it out nice and thin and even.
Then shape the main folds into the piece prior to fixing to the mini. Also get some clay shapers from your local arts and craft shop, better than dental tools and use water too. This works well for tabbards, as well as cloaks, straps and such like too.
Keep at it, when I first used millput then GS I could bearly fill the gaps I needed, now I'm sculpting new helmets, heads, limbs, weapons and such like, it's addictive once you gain your confidence too.
Good luck mate
The Commander