If you have an iPhone 6, 6+, an iPad Air 2 and newer, or a Stratus 2S, you will be able to see Pressure Altitude in the Heads Up Display (HUD) within ForeFlight Mobile. What is displayed will be the pressure altitude as sensed by the barometer sensor. This altitude, however, is currently based on the standard atmosphere of 29.92 in hg (1013.25 Hpa). Because of this, pressure altitude will not always indicate your correct altitude above Sea Level. The barometric pressure can not be used to adjust GPS altitude displayed within ForeFlight Mobile.
Of interest, the altimeter in your aircraft is measuring a pressure level, not necessarily altitude above mean sea level. GPS altitude is a geometric altitude, which is a measurement of your actual altitude above the earth.
Pressure altitude and GPS altitude differ more and more the higher you climb (as they are measuring different things). To learn more, here is an well written article on pressure altitude vs. GPS altitude: