1992 Nissan Sentra - oil and water mixed

My cars oil and water mixed but didnt overheat …i replaced the gasket …but still does the same …i felt my car jerking …then check my water and it was empty but the motor was low in temp …so what is going on

You likely have a cracked head or block. That assumes you installed the gasket properly. Pressure test the cooling system. A failure will lead you to a hot compression test. It will identify if the heat is cracked if the compression is off both dry and wet. Allows you to look at the plugs - a cracked head will show a very clean plug.

If the compression test is ok, suspect a cracked block. At that point, a replacement engine is in order

The coolant temperature gauge was low because there was no coolant to measure the temp of.

Your gauge actually measures coolant temp, not engine temp. If your coolant is low enough to not cover the coolant sensor, there is nothing for it to measure. Your engine can be so hot that the exhaust manifolds are glowing yellow and dripping and the gauge will never show it.