I use cooler lines to pump old atf out, usually. There's very little pressure on those lines typically. I don't expect that to be different for your truck.
I pump out 2-3 litres at a time, refill with clean fluid, pump out again etc until the fluid becomes clean. That's the main benefit, it also replaces the fluid in the converter. If the fluid is still clean enough, regularly performed pan drains work aswell.
From what I can see, 0-30 psi gauge would be fine, but I'm not sure you can use the pressuret to determine filter loading.
Well, the wix trans filter opens its bypass at 7-9 psi.
So put the gauge on the input side of filter.
The drain side back to trans has about no fluid resistance.
As the filter slowly clogs, the pressures should rise until it hits 7 -9 psi, then you will know the filter bypass has opened and time to change, is my thinking.