There are different models of the gripshift -- I have the most recent "Wave" variant. Never had any problems with my hand slipping on it, no matter the weather. (I wear gloves all the time, so that may help.)
What I have done, to ease any strain on thumb and wrist, is to cant the Thorn accessory bar (on which my shifter is mounted) backwards about 25 - 30° from the "east-west" position across the axis of the bike. That lets me grasp the shifter more or less end-on, so that the fleshy part of my thumb is on the back/outer part, and my fingers cover the numbers and the bracket holding the shifter. That makes it more comfortable to use, and (I find), easier to grip.
But y'know, if you just don't like the rotary item, then there are thumbies out there.