Yes, it is 1/4" male NPT bottom mount.
Sounds like you might have a bad defective gauge - it should already read at zero when you take it out of the box before installing.
The size is 1/4” pipe. You’ll need a 1/4-18 NPT tap. Yes, it will look more like 1/2” diameter because 1/4” inside diameter pipe is about 1/2” outside. Here’s a link to the item you need. 1/4 in. -18 Carbon Steel NPT Pipe Tap https://www.homedepot.com/p/304632091
Not filled with glycerin
The item did not have a bolt to attach to a hose bib. I will be returning this item.
FIVE GALLON TANK ON A PROGRESSIVE PUMP
Yes I used it for my well.
Yes, do not have to disconnect
Yes it will work on outside galvanized water tank. Ours will be exposed to weather conditions but we do not intend to build an enclosure so time will tell.
No. It is designed for water systems.