Cheers

The more of the yellow stuff you have, the softer it'll be. But I seriously don't think that's your problem mate, it shouldn't matter if... (tension...drums...oh, oh, what...tell) you just waited a bit longer.
My guess is that you mix the GS, shape it, apply it and then try to add details to it before it has cured.
Waiting is to GS like the Warp is to Chaos. Essential.
Try to do like this.
Mix it and put it on a flat surface, could be plastic or whatever. Make sure the area is either a bit damp or have been treated with vaseline or something similar. Make it flat using something...could be a fingertip (not really good) the handle of your scalpel (roll it over like when baking stuff...biscuits and such)
Wait
Wait
take a cup of coffe
wait some more
draw a deep breath
finish your waiting
Gently pry the cloak (it should by now be semi-cured) from the base-thingie and apply it to the model. Make the basic shape, shape as in movement, nothing fancy like..well fancy stuff, details and such.
Make sure the model rests in such a manner the cloak can neither move nor get pressed out of shape.
Go to bed.
Sleep.
Wake up and do day-like things (work, study..you know)
When evening comes pick you cloaked model up and add details such as detailed stuff (rims, jewels) and add some GS-sausages to create foldings and wear.
I know it takes a lot of patience, and I've heard about people building GS-ovens (basically a coffecup (this should be empty) and a light (lamp, not a candle) of some sort. They then put the model in the cup and put the light just at the opening. This will make the GS cure faster, but I think it also adds some of that much hated bulging and less is in this case far better IMHO.)
Oh, and finally cut the edges sharp with a sharp knife.