With one exception, the manifold pressure gauge will be repairable, at a suitably qualified facility. The exception is an instrument of a vintage of 50 years or so, whose dial and/or pointer are painted with radioactive paint so as to glow. Those instruments are generally refused by instrument shops these days. Be careful even throwing them out, they are considered hazardous material. These are recognizable by pointers and/or graduations and numbers which are not bright white. They may be slightly greeny, or dingy brownish. DO NOT retain such an instrument for the mantle, it is a hazard out of the aircraft. If the markings are bright white, you'll be fine.
As for the problem at hand, yes, the internal mechanizm can gum up and slip. They are nearly always able to be cleaned and made to work well again. There are no non-metallic parts in most of these instruments, so nothing perishes in there, just gums up (small amounts of gasoline do get in there, and leave gunk over time). There is also a tiny adjustable orifice at the inlet to the instrument, which can gum up, though this will cause sticking, not an over indication.
Do not rely on the instrument for any further flying until fixed, very particularly if the aircrft is a turbo. Have a shop remove the instrument and send it for overhaul. While they're in there, the manifold pressure line should be blown out to the engine. Other than that, if the line is in good shape, there's nothing else to worry about.
Pilot DAR