Medicine Hat's apparently doing pretty well in terms of population growth partly because it has large natural gas reserves around it, but it's not exactly booming like GP or Red Deer.
The Hat's population is currently 56,997 according to its 2007 census. GP could reach this many people in as little as just a couple of years, and may probably surpass Medicine Hat in population sometimes before 2015 or even 2012.
I know that Medicine Hat and Red Deer were both about the same size (about 30,000) way back around 1975. GP was only around 17,000 at that time. But as the oil boom in Alberta picked up throughout the 1970s and early 1980s, Red Deer's growth began to accelerate and pulled further away from Medicine Hat in population.
Red Deer now has 85,706 people as of its 2007 census, so the gap has now widened to 28,709.
Medicine Hat historical population growth:
Red Deer historical population growth:
Grande Prairie historical population growth and projections: