I also have a GMP100-3 furnace and the air has always been just luke warm. Maybe that is why my gas bills are 300 to 400 a month. The gas pressure you are talking about is the incoming or out of the gas valve? What is the easest way to check it myself?

First you need special tools to check and set the pressure. It would be best left to a professional especially when playing with gas.
If you gas bills are high have them service the entire furnace. This way you will know it is working at maximum efficiency. Change the air filter often or when dirty.

I have most of the equipment that is needed and have used them before.

Then it should not be a problem since if you have the tools you know how to use them. Just remove the pressure tap out of the gas valve an hook up the digital or WC pressure tool. Then look on the furnace or the factory pressure setting and adjust to it. Piece of cake . The cleaning you can do yourself also. Now all you have to do is get it done.