It's actually a Chinese translation of the Japanese leathercraft book. The literal Chinese translation of the gel is "fleshside processing agent," which I believe refers to the burnishing gum Tragacanth replacement, which is sold by Goodsjapan http://www.goodsjapan.jp/item/leather-craft-items-leather-fi/seiwa-leathercraft-burnishing-/lid=29749117
Since it's a gum trag replacement, I suppose you can just use gum trag.