That gauge doesn't really tell you how much gas is in the tank. It will pretty much read the same from the time it is full until just before it is empty. I wouldn't bother replacing it.
It is better to judge the contents by the weight of the bottle.
Sounds like a worthless gauge, if it always reads the same until it's too late.
Here's a quote from Tom that may help explain it better:
CO2 tanks have liquid in them when they are full, and maintain a fairly constant pressure inside the tank as long as the liquid is there. The pressure they have depends on the temperature of the tank. PV=nRT R is a constant. In the tank, volume (V) is mostly constant, but a you use the gas the V will increase over time as the liquid turns to gas. n is the amount of gas, which will increase slightly as the volume increases. The temperature you keep the tank can vary, so this will affect the pressure of the gas in the tank slightly, but as the pressure increases the gas is more likely to turn into liquid. So anyway, the bottom line is that as long as there is liquid in the tank the reading on the primary, high-pressure, gauge will remain pretty much where it started. It is only when the tank is empty of liquid that the needle on that gauge starts to move.
<edit to correct to Tom from Tim. Sorry, Tom>