If your saddle fits properly you shouldn't need one.
If your saddle dosen't fit properly then spend your money on getting a saddle that does fit.
Gel pads sound like a nice idea but really they are just another product on the market to part people from their money, breathable this - shock absorbant that , comfy - it's all just sales pitch.
I don't have one - and I will never have one ( and I've had about 40 horses - currently down to 8 ) .
I also know a registered master saddle fitter who dosen't carry them - and will only get one for a customer if they really insist - and he knows what he is doing.