I Have A Gt5, on one of my bikes:
I put an Extra seat Post all the way to the bottom. I Measured and then made my choice. Cause once you put it in it is not coming out. I used a pipe cutter to get that clean cut. Then I used a rubber mallet, hit your seat post on top of it, to pound it down, of corse take the seat off 1st. It double thickins the Frame seat post tube where the motor mounts to. It helped a bit.
Here is one of my bikes on Youtube it has a double seat post but you cannot see it unless you take the seat & post off & get a flashlight and look down in there yes it helps to make it more solid: