There's a bucket load of different tyre pressure guages out there. The main 2 types are, Pre set valves, which can be pre set to (eg 18 PSI) and when they're done, the air stops coming out. The other is the Tyre deflater with a PSI guage. This allows you to stop at any pressure you desire, and is handy for dropping a couple of PSI when needed. The benefit of a tyre deflater with a guage connected to it, is that you can try different pressures depending on the terrain (eg from 38 to 26 to 15 PSI)
The other advantage is the 4 PSI rule and in some towing caes a 6 PSI rule. Check your tyres when cold, like at the start of the day before you drive your chariot, then drive it for an hour, jump out and check your pressure again.
If the tyres have gone up 4PSI then starting pressure was correct when you initially set your road tyre pressure. If they are more than 4 PSI or less than 4 PSI then your starting pressure was either under or over inflated respectively.
If you're using tyre pressure guages for deflating on different terrain, then you really need to get yourself an air compressor too. When you get back on the tarmac with low pressures, pull over and pump your tyres back up. If you don't get some air back in, you'll probably heat the side walls of your tyres up and they will most possiably explode when they reach a certain tempreture.
Different terrain requires different tyre pressures, get to know your vehicle better by trying a few while you're travelling. There are endless forums and published content, so read up as much information you can find and save those tyres.