Pressure gauges, like all measuring tools, have a tendency to wear over time and may become less accurate. Because pressure gauges are often manufactured to accurately read only the middle values (the low and high ends of the scale provide less accurate readings), just "zeroing" your pressure gauge may not ensure a good reading. Before you run out and purchase a new pressure gauge, consider calibrating it yourself.
Measure the pressure of something with the questionable gauge. Then use an accurate gauge to measure the pressure of the same thing. Record your results.
Cross-check your gauge with another one on several items of varying pressures, recording all results.
Change the dial or make a note. Some pressure gauges allow you to adjust the dial to calibrate, but most do not. Instead of forcing the gauge and potentially breaking it, make a note such as "reads 5 psi short" on the gauge.
Go to an ISO-certified facility and ask to have your gauge calibrated. Facilities that have ISO certification and use pressure gauges often have the right equipment for very accurate testing and calibration. Do not rely on a "zero" reading. Just because the needle does not rest on zero when the gauge is not in use does not necessarily mean the gauge is inaccurate.