Yes, Absorbant Glass Mat. They have fiberglass matting in between the lead plates which acts like a sponge to soak up the electrolyte and hold it in place. Plates can be made thinner (since the matting keeps them from touching and shorting out, a common death of flooded batteries) so it's more efficient.
Gel batteries use lead/acid as well, but the electrolyte is modified to be in a gel form rather than fully liquid. I'm not aware of anyone making actual gel lead acid batteries in the sizes we need, and I don't think they are great at the high current draws needed for starting and winching, but could be wrong.
AGM fixes a lot of the issues traditional flooded lead acid batteries have. They're sealed, non-spillable, greater resistance to heat and cold, can be used on their sides or even upside down, far more resistant to vibrations and abuse, and they don't cost a whole lot more these days. I picked up a group 31 for less than $200 from Sam's Club and it's been going strong, even with infrequent use of the H3.
https://www.samsclub.com/p/duracell...nd-rv-battery-group-size-31dtmagm/prod3590232
