The gels are in Portal 2 because of the Digipen student game Tag: The Power of paint:
https://www.digipen.edu/studentprojects/tag/
As you probably know, the core team of Portal 1 came from Digipen. They made a student game using portals, and Valve swooped in and hired them all. So then TAG comes out a while later, and draws lots of Portal comparisons (manipulating the environment to solve puzzles, etc.) Valve swooped in and hired all of
them too. And suddenly you have surface gels in Portal 2.
But in tag there were FOUR gels. The three that made it into Portal 2, and a fourth sticky gel that allows you to walk on walls, ceiling, etc. It was (obviously) eventually removed from Portal 2.
Edit: It's interesting to me that Portal 2 existed for a long time (over a year!) without gels at all, considering what a huge part of the final game they are.