You've done what I did, and are left with the same questions I have.
My theory is the gage is set up as a qtr bridge circuit, which can be highly sensitive to milliohm variations - which most DVMs won't pick up.
It definitely doesn't act as a switch. Put the sender on a test rig and increment the applied pressure and the gauge will move accordingly. As best as I can tell, the gauge electronics are just very sensitive to very small resistance changes.
I've seen several LS swaps that use an adapter to...