The GAP is the "Grade Adjusted Pace".  It is a pace that factors in the grades that split is covering.  So, if you are running uphill, your actual time may be 10 min/mile, but it does a calculation (I don't know the specifics) to figure out what that pace would equate to if it was on flat ground.  

Hey @Kellydoc, 
To add on to what @anchskier noted; Grade Adjusted Pace takes into account the steepness of terrain during your runs and estimates an equivalent pace on flat land. Because running uphill requires extra work, your Grade Adjusted Pace on ascents will be faster than your actual pace. And similarly, when running downhill, your Grade Adjusted Pace will tend to be slower than your actual pace.
 
The difference between GAP and actual pace generally becomes larger as the grade steepens. Athlete data has shown that the downhill adjustment peaks around -10%, after which it becomes slightly less extreme. Note that GAP does not account for the technical difficulty or condition of the terrain. Learn more about the determination of the GAP model from real-world data, here.
 
Feel free to read more about the GAP here: https://support.strava.com/hc/en-us/articles/216917067-Grade-Adjusted-Pace-GAP 

Is GAP based on elapsed pace or moving pace?