What exactly is the material inside a gel mousepad?

Not too sure on the specifics you’ve asked for. The gel used however is normally silicon. The stuff they used to put in breast implants. Go-on give it a squeeze!

My understanding is that it’s actually a urethane rubber—silicone rubber is much more expensive—you can buy A-B urethane rubber (and silicone) from http://www.smooth-on.com/

Generally a very low Shore hardness (something like 10A) will give you the squishiness Here’s a good table (scroll down to Durometer hardness): http://www.mcmaster.com/#about-rubber-durometer-hardness/=621kz6

If you just want to play with some stuff, you could try this:
PMC 121–30 WET – Trial Size from http://www.smooth-on.com/Urethane-Rubber-an/c6_1117_1144/index.html?catdepth=1

It’ll be a bit harder than the goo in gel mousepads… you could try VytaFlex 10A:
http://www.smooth-on.com/Urethane-Rubber-an/c6_1117_1142/index.html?catdepth=1

If you wanted to try a translucent silicone, you could try the Mold Max 10 here:
http://www.smooth-on.com/Silicone-Rubber-an/c2_1113_1135/index.html?catdepth=1

Silicone is much harder to work with, and you really want a vacuum chamber to degass it and must use a scale to mix properly.

@grumpyfish Thanks a lot for your answer! If you were to make a mat with some sort of gel that moulds to the shape of your body, therefore straightening your back, what material would you use?

I don’t know if this would work, but:

What I’d do is take a cast of the back with Alginate—Alginate is skin-friendly, but doesn’t make stable moulds for more than a few hours.

Reverse cast a plaster version of the back, and while the plaster is still a little soft, make any changes to it that would cause it to straighten the back. Then use the PMC 121–30 WET (linked above), which is a 30A durometer rubber to make a permanent negative mould.

However, if you’re just looking to make something that pushes back against your body, check out Memory Foam—it’s pretty cool stuff.

The same stuff that’s in Pamela Anderson’s breasts.

@bvdshec17 have you looked at visco elastic foam? . . ..aka memory foam.