I use Dermalogica Exfoliant Accelerator 35, which is a 30% Lactic Acid. Not a gel, but it spreads nice, isn't overly stingy and gives a great result. You can mix it with other exfoliators, or use alone.
I love Image skincare lightening peel! it has 50% blend of lactic and kojic. Aloe based gel and you can boost it with retinol and more kojic for higher streght.