Calibrate barometer using QNH or SLP?

5829294 said:

using the QNH value provide by nearest METAR station. But during the calibration the watch asks for "Sea Level Pressure".

QNH IS calibrated seal level pressure !

Not a simple answer as those two are derived from two different methods—QNH relating only to an aircraft's altitude and SLP corrected for both elevation and 12-hour mean temperature of the station. In high altitude places like KDEN the difference between the two could be large. Generally QNH value should work better on the Garmin device as you could assume it functions similar to an aircraft's altimeter scale.

Reference: https://www.wingsbywerntz.com/520-metar-slp-sea-level-pressure