Gorilla Mats can be a little slippery when they are brand new.  We call this the "break in" period and it will definitely get better with use!  This is the nature of the ultra-durable top layer of the mat.  Also, hopefully you take comfort in knowing that the sweat and moisture isn't absorbing into the mat.
You can speed up the break in period by cleaning the mat with a small amount of mild dish soap and warm water.  Or lay it out in a high traffic area for a week or two.