These recharge kits should be outlawed and many do not provide pure R-134a, but seem to have a bug about adding sealers that can cause more damage than good. Precise operating pressures are imperative for proper AC operation and you don't get that with a low side gauge I wouldn't even trust to measure my tire pressures with. Another factor in particular with a 1994 GM truck, your system could still be R-12 as really a mix up in these years. And the chemistry of toping off an R-12 system with R-134a results in extremely high pressures much greater than R-12 or even R-134a is capable of. If you don't have clutch or other electrical problems, by adding R-134a to an R-12 system, it can blow up in your face.
What I hate about these kits the most, they assume and try to convince you that your only problem is that your system is low on charge. And you already told us you do not know the difference between static and operating pressure. Don't you think you should learn about AC systems first before even playing with one?
GM has more AC systems than you can shake a stick at, best to get an alldata to learn what you have. Most basic is 12V from the climate control through the cycling switch, through a high pressure cut-off switch, to the clutch. With the cycling switch switching the full inductive load of the clutch coil, they don't last very long. Other extreme is a microcontroller based system with sensors all over the place. Another common characteristic is that crazy interference fit clutch hub that can either creep in or out depending on it's mood, so you either end up with drag or a clutch that won't engage at all due to excessive gap.
Ha, but none of these are problems, right? Has to be that your charge is low.