Corrected pace on your Garmin watch
Opposed to the detailed analysis carried out by running power meters like RunPowerModel, the principle of the grade-adjusted pace is rather simple. Depending on the current gradient you're running on, a correction is applied to your pace to adjust for the changed effort when going up- or downhill.
You can choose between displaying
the current GAP
the one of the current lap
the total average
All information is of course also saved to ConnectIQ. In addition, the data field comes with a battery-saving mode, which, if activated, updates the displayed results only every few seconds.
Last but not least: "GAP - Grade-Adjusted Pace" is free!
Equivalent Flat Pace - The Next-Level GAP Alternative?
Despite its advantages, the grade-adjusted pace has a major downside: The applied correction only depends on the inclination of the path and neglects its surface. Be it a tarmac road or a technical single trail, the GAP cannot differentiate - but the latter is of course much harder to run on.
An alternative quantity which improves upon that situation is the Equivalent Flat Pace (EFP), a new measure which is calculated by RunPowerModel. It uses your current running power level to estimate the corresponding pace you'd run on a flat, non-technical road. And as RunPowerModel takes into account technical sections, the EFP is definitely an interesting alternative especially for trail runners!
Please note that the current EFP implementation takes some time until it delivers stable results. It can furthermore give biased results for pure uphill runs. That being said, in general it should work quite well!