What airfoil design does the MQ-9 Reaper UAV use?
The wing root of the General Atomics RQ-9 is the Drela GW-19/GW-25. 

The wing tip airfoil is the Drela GW-27.

Because of their application, you will likely have difficulty finding study results of vortex generators used on that blended airfoil. There are papers, but I have only seen them in the DTIC library, and I do not recall the classification of them. However, the aircraft has been exported, so you may find some open literature on the airfoil, and the adaptations and optimization of it.