The pressure gauge is probably bad, pool companies tend to use cheap gauges that don't hold up in corrosive environments, I can't really blame them for this as a high quality corrosion resistant gauge would likely cost as much as the filter. As a result these cheap gauges die every few years, the good news is they are cheap ($5 on ebay), if you want to you can replace it with a better quality gauge, these can often be found new old stock on ebay also at a fraction of their original cost.
Ike
p.s. to answer your question they are usually either a 1/4 or 1/8 inch NPT fitting pressure gauge, any water rated gauge with the right pressure range will work, if the gauge is marked WAG, it will work as that means Water, Air, Gas rated..