The hose may be stuck, kind of 'glued 'to the fitting.. To loosen it, you have a few options. After removing the zip tie, use a pliers and gently rotate it back and forth to break the seal. Option two is to use a thin bent tool like a angled pick under the lip and then twist.
If you have enough slack, you can use a razor or knife to cut the hose. You can then split the end left on the gauge to get it off.
I am pretty sure that is a water pressure feed from the pitot tube for the speedo function. Note that if you remove the hose and while waiting for the gauge to be installed, should you take the boat for a spin, the tube may spray sea water.