If there is a restriction on the suction side the pressure and flow would be low. Gauges are low in mass and prone to freeze damage when everything else can survive. They run about $6 or so at a pool store so it's simple to eliminate that item. Something else I just realized, 45-50 psi probably cannot be reached with a centrifugal pump even with a fully blocked output. My 1 HP won't quite hit 30. Also, bet the max working pressure on the sand filter is 50 psi. If you look at the tank when you kick the pump on you'll clearly notice the thing expanding which is normal. More reason to suspect the gauge first.
Al