Cannot turn gas valve. It is stuck
After another attempt, I succeeded. I reoriented the crescent wrench in the opposite direction (concave side away from me) and pulled towards me. I believe this gave me more leverage is it was significantly easier to turn. 

Other ideas that I ended up not trying but may help as well:


Getting an extra large crescent wrench (expensive). 
Getting a tool with linear slot that fits over the valve's knob which appears to be created for opening valves. 
(Rob's suggestion) As the valve is located outdoors, there was a bit of rust or dirt that can add friction to the valve. Using PB Blaster or Liquid Wrench can help loosen things up. (NB: please check if this is safe)