I use it at Poppy's withers, then put on the saddle pad, then half pad.
Here is Poppy's former half pad by Beval.
As you can see it is old and just needs to be retired. RIP Beval pad. It is thick and really did a good job with saddle fit. When combined with the Gel-Eze I had no saddle slipping so I wanted a similar sized new half pad.
I switch between a Professional Choice girth and a fuzzy fleece one with Poppy. I use the fleece one for shows mainly. The girth I use doesn't help the slipping. When I use the Gel-Eze there is no slipping with whatever girth or half pad I am using.
I am a fan of minimal tack/gadgets. I only use a running martingale in the show ring. So I love that the Gel-Eze is small and hidden under the saddle pad. It is also small enough that it would fit under a hunter pad.
There's my sales pitch. If you have any saddle slipping try the Gel-Eze!