“Flex might be overkill for a 172” - meaning, presumably, just too expensive for recreational GA.
Which is a shame; I’ve been looking for a long time for an affordable 2" altimeter for use as a backup to my EFIS. I don’t expect I’m the only one… guess I will have to just keep looking.
My backup GPS (Garmin 696) has altitude as a displayed parameter. In a pinch (emergency) I think that would work just fine.
I wouldn’t depend on GPS altitude except in an emergency (presumably in IMC) and not even then unless I was over flat terrain with WAAS available at the airport I was landing. The GPS altitude is based on the ellipsoid, an approximation of the shape of the Earth, meaning that unless your GPS also has an accurate terrain database, that altitude could be off by hundreds of feet.
Also, the loss of a calibrated altimeter in IMC is automatically an emergency, whereas having a backup calibrated altimeter would allow you to continue to your destination. (Of course, if your primary altimeter doesn’t fail hard, you’ll also need a second backup altimeter, and pitot/static system, to break the tie.)
So what the OP really needs is an accurate and inexpensive radar altimeter for his 172. 
I have found the GPS altimeter reading not too bad, not too bad at all. About 100 ft out. It is a good cross check when the other fails. I had one failure that snuck up at me, it was due to ‘stiction’ when my very smooth Jabiru engine was too smooth to rattle free the altimeter needle and gave me a falsely high height.