Don't gas fittings have threads the opposite way? To prevent people
from accidentally/stupidly attaching gas pipes to water pipes? Or am
I wrong about this?

Anyway, I stopped believing in trusting advice given in mom & pop
hardware stores. The first time the clerk told me, when I was buying
lamp cord, which color was for positive and which for negative... it
took me ten minutes to explain alternating current to him.

The second time, in a different store, I was looking for ground wire
for an outlet (replacing old two-wire outlets with three-wire,
grounded, outlets). I told him I wanted 12 gauge green wire. He kept
telling me that color didn't matter. And I insisted that I was pretty
sure that either green or uninsulated was the standard for ground.

I don't like my local Home Depot (and I'm a shareholder) but I don't
see a greater level of service/advice from my local hardware stores.
And when I bought a new house and wanted cover plates for outlets and
light switches, they were around 12 cents at HD and 79 cents at the
local store.