Grade Adjusted Pace (GAP), sometimes called effort-adjusted pace or equivalent flat pace, converts the actual running pace on hills to what it would have been on flat ground at the same effort level.
Running uphill requires more energy at any given pace. Running downhill is faster for the same effort — up to a point, after which steep downhills become inefficient and hard on the quads. GAP accounts for this: if you ran a 5:00 min/km segment on a 5% uphill, your GAP might be 4:30 min/km — meaning you were working at the effort level of 4:30 min/km flat pace.
GAP is calculated using the relationship between gradient and metabolic cost derived from energy expenditure studies. Strava's GAP implementation uses a formula based on a combination of energy expenditure research; Garmin and other platforms use similar but not identical models, which is why GAP numbers differ between platforms.
GAP is most useful for training analysis in hilly terrain — comparing efforts between flat and hilly days, or ensuring you're training at appropriate intensity on varied terrain.