A pipe wrench.

Often larger than you would think (like at least 18-24 inches).
Be prepared for damaged (corroded) threads that will require special materials to re-seal.

I got a 36" pipe wrench (the entire neighborhood has borrowed this at one time or another).

Figured a pipe plug, some pipe dope and teflon tape.

So maybe I should have a pipe tap on hand?

Even a tap may be useless if themetal threads have corroded away.

There are a number of compounds available (the one I am familiar with is a powder0 that are mixed with water to a thick paste, then smeared over the area of damaged or missing treads, then a new NPT male is screwed in to form the threads.

Remove the temporarily fitting carefully (often just a nipple long enough to easily handle), then use pipe dope on the gauge.

For a gauge it should not mater much if is is a little cocked, but for pipe runs some temporary strapping ay be needed to hold the nipple correctly.

Well I lucked out. The gauge was brass, 7/8" square fitting. An open end wrench and a 4' piece of pipe - and it came right out!

I'm glad it happened - cause I saw that the flues in the boiler were completely blocked. Really didn't want to clean a furnance in January, in Minnesota, but it was 32 degrees today - so it all worked out.

Thanks for the consult.

"Really didn't want to clean a furnance in January, in Minnesota, but it was 32 degrees today - so it all worked out. "

Practically a heat wave!

Glad it worked out.