https://greatbritishequinery.com/shop/ols/products/vip-equestrian-saddle-pad
Full disclosure- if you google VIP pads, they use them directly on the horse’s back. I do not, I have a friend who does but I prefer it over my pad. 
and I stand corrected- it isn’t “gel” it is “visco elastic polymer” ( gel ha ha) . And I don’t use it directly on their back, I put it between my saddle and the pad.
I have the black pad because when I bought it last year it was all they had. I put my square pad or fitted on first, I put the gel bubbles down against the pad and the smooth side against my saddle ( most people do the gel bubble against the saddle and smooth side down- I like it better gel down).
I was going to buy the white one- buy you can’t even see the black with my saddle on. I have had it over a year and LOVE it. 
I’ll upload a picture tonight with my saddle on it.
What I love about it- no cover to wash, stores easily, doesn’t add bulk, you can’t tell it is there, I feel a difference when I ride without it and I am sure my horse does as well.
ETA- you can order VIPs not just from the link I sent above- that is the saddlery I primarily use now because they are a small business, they are fantastic to work with, when a square pad of mine got a tear they immediately replaced it, and I love their velvet type square pads for schooling.
I have 3 of them now and adore them.