I think I played with Gus and Bison a couple of times. If you're looking for some good, cheap, chaotic co-op games besides L4D, I recommend:
1) Strange Brigade. 1930s style pulp action where to take stiff upper lip Brits soldiers, mad scientist types, occulitsts, and a Brummie lesbian mechanic and fight mummies.
2) Vermintide 2. Melee combat focused hack n slash set in the Warhammer fantasy universe. Good fun. I am bummed that they increased the height of the Dwarf so he can now see over the top of the wheat on the farm level. I thought it was hilarious that he couldn't see anything at all and gad to be guided through it when it was first released.
3) World War Z. Zombie shooter similar to L4D but much more gun focused. The mass zombie waves in that game are very cool.