Thought it might be helpful to post an update.
Turns out the gauge was faulty. The new one from TFTkits was great, and after putting it in it read 19PSI. So, I just have an extra set of cartridge filters in the attic, guess I should've been a little more patient!
On the other hand, our heater still isn't working. I can hear it firing up but won't stay on (screen is blank too). PB thinks it might be a fuse, so we'll see.
Anyway thanks for the help on the cartridge filter problem and pressure gauge!