I suppose you could dilute the gylcerol in your pre-workout shake. GMS is a little more convenient since it comes in a solid form. That's about the only rationale I can think of for using GMS: convenience.
I'm personally not the biggest fan of the stuff. While it offers some great things in terms of pumps and vascularity via super-hydration, it offers little to no benefit in terms of anabolic effect or performance enhancement.
Very basically, and part of the reason why the liquid form can be dangerous, glycerol sucks the moisture out of anything and everything it comes in contact with. This can be from in the air (if you leave the container open). When in the body, it sucks the water from your cells (i.e. fat cells, muscle cells, you name it cells), and puts it into your bloodstream. Then end result: increased blood volume. Increased blood volume will increase sensitivity to nitric oxide, since there is more blood to travel through dilated blood vessels.
Use with caution when taking with pre-workout products, such as those containing stimulants, as this could create a not-so-good increase in blood pressure. Think of it like a balloon. Glycerol increases blood volume = blowing up the balloon = increases pressure. Stimulants can be vasoconstrictive = pushing down on the balloon = increases pressure. So if you were to blow up a balloon and sit on it....POP!! it could put you in a hypertensive state.