I have some training on Combat Gauze (CG). A close friend is a MD who has done live tissue training with it.

We were taught that the main enemy of CG is moisture. Kaolin is the main ingredient of the hemostatic embedded in the gauze, and its interaction with the water in blood is a big part of why CG works. If the CG package gets ripped or has pinholes, it can absorb moisture from the air over time which will decrease the effectiveness. Keep the CG package protected inside another poly bag (loose, not vacuumed) or even just a heavy ZipLoc bag to enhance the wrapper's durability.

If your CG package looks beat up and has been riding in your blowout kit for 3 years, you should consider replacing it. But AFAIK it is not going to suddenly be inert the day after the expiry date.

To reiterate others' good advice, training is absolutely key, especially proper wound packing. Note that a good wound pack doesn't have to have CG or any other advanced hemostatic. In fact for many years a Special Forces medic's chief packing tools were big fluffy rolls of Kerlix and ACE bandages. If you had to, you could cut up a shirt or a set of curtains and pack that wound (after gaining initial bleed control as best you can). CG is just a modern adjunct that gives you an edge, but it is an edge worth having IMO.

Useful links:

Israeli military's experience with CG in a real operation

NAEMT TCCC complete curriculum