Zéfal Air Pump gauge not working
I've had something similar on my Topeak Road Morph; in fact you remind me I need to strip the gauge again.  My pump stays on the frame in all weathers.  Maybe yours does too and suffers in the same way.
What I've found is that the gauge can get muddy water in it, with enough grit to make the moving parts stick, either completely or partially (the latter cost me a tyre when I overinflated it and blew out the sidewall).  It wasn't seals that were the problem, but the spring and sliding indicator.  A good rinse and re-grease did the job for a year or so.
Even if you can't fix it and the gauge is in the head, you may be able to replace the entire head, either with an identical one (probably the best idea for portable pumps as you don't want it flapping around) or a similar one (on a track pump).  On my track pump the gauge isn't on the head but on the pump itself - it does however use a standard pressure-fitting thread so any pressure gauge of the right range would do.