GPS altitude is very useful because you can level off for trouble shooting with reference to it, specially when you don't rely on your baro altitude indications (not even the FPV).
The technique to fly the UNRELIABLE SPEED procedure is to maintain flight path (level flight) and then see what happens with pitch in order to adjust the thrust required. So you need too know if you are maintaining level flight (if you don't, the whole thing is of little use) and GPS altitude gives you that.
Also, at lower altitudes below transition level it will give you an approximate altitude, but it is advisory only, so to speak.
Groundspeed is no so useful, but at least you can see if you are accelerating or decelerating. of course angle of attack is a much better information to have in the left side of the PFD