"Explosive gel" refers to a substance that can explode when triggered, often used in controlled demolitions or by superhero characters to blow up objects or enemies. Full definition
Holding L2 / LT and pressing square / X for example makes Batman shoot a large amount of explosive gel to blow up in a wide blast radius to stun or knockout enemies. (jamiex66.com)
Rig a nearby wall with explosive gel and then cause debris to rain down on foes? (pressthebuttons.com)
It's these Predator areas that capture the power fantasy of being Batman so perfectly, allowing you to stalk your prey from the shadows using a variety of takedown moves to thin the herd, striking from around corners, grabbing enemies through windows, laying traps using explosive gel and stringing thugs up from the rafters. (wolfsgamingblog.com)