Knee gaskets are neoprene sleeves that you wear under your knee pads to give your knees a bit more stability and support.
After wearing Pro-Tec Street knee pads for about a year, I added Gladiator knee gaskets under them and felt the combination was definitely better. For the price of Pro-Tecs plus Gladiators, of course, you could get comparable protection with heavier duty knee pads like Rectors or 187s. I still wear Gladiators under my Rectors, though. I tried them without and worried the whole time that they were about to slip. I did size my Rectors with the gaskets included, so perhaps they're a bit too big without.
The other brand of knee gaskets that I know about are TSGs, which have an aluminum insert that goes up the side of your knee for extra support. The aluminum bit, though, eventually pokes through, and it's not polite to have metal protrusions. TSGs are especially recommended if you have weak or injured knees.
