Gameday isn't a radar reading. It's the PitchFX pitch tracker that takes 60 high-speed photos per second during the trajectory of the pitch, measuring velocity, spin, and break relative to an x and y axis. Radar readings can be misleading because it depends on where in the trajectory it is picked up. PitchFX tells you how fast it left the hand and how fast it was going when it got to the plate.
A site, brooksbaseball.net, mines data directly from mlb.com and breaks down each pitcher's performance in each game.