ATAB contains a quaternary amine and a terminal carbon-carbon double bond. The polyacrylamide reaction links acrylamide by their double bonds forming the polymer. The double bond on ATAB can be incorporated into the growing polymer chain, and the quaternary amine holds a positive charge, making a positively charged polymer.
The immobilines mentioned are probably just a weak acid or weak base group with a terminal double bond like ATAB or acrlyamide. They get trapped into the polymer and if you pour the gel with a pH gradient, they pick up charges according to the pH. I haven't found a good concrete immobiline example, I can't access that second article you linked.
I am away from my good computer and cannot make chemical structures right now.