I've used a gauge, and never had a problem.
Liquid normally is not cold, so it won't freeze the seals in the gauge. Liquid does get cold when it expands, but with a siphon, that happens OUTSIDE the gun.
Gauges are not very useful with co2, and even less useful with siphons.
The problem is:
1- FPS will jump 50fps once the valve is flooded, but the gauge will still read the same pressure
2- FPS will not drop when rapid firing, but the pressure in the tank will drop.
3- FPS will not rise in hot weather, but the pressure in the tank will rise.
So, the gauge ends up only being useful in telling when the tank is empty. And with a siphon, you will already know that because your FPS will suddenly drop 50fps near the end of the tank, and you will no longer see a vapor cloud.
Generally, set the FPS where you want it, and it will stay there as long as there is liquid in the valve. I've never adjusted the FPS on my 12 year old MegaZ.