I’m finishing an old (1930) empty basement and have a random pipe coming out of the floor. I have no clue what it is or what it went to as it sits alone in the space. It is metallic (tested with a magnet). It’s currently capped off and did have threading. I would say it is 3/4in thick. I am wondering if I can cut it off and recap or if it’s not able to be cut, if I should build it into a wall?
Thanks in advance!!! 
In a basement, galvanized pipe like that could have been a pipe feeding oil from an underground tank to an oil heater that has long ago been removed and replaced with something else. I had one like that and figured it out by looking at where an old chimney had been removed by looking at the sub-floor structures made to accommodate it. I didn't know exactly where the pipe lead until I accidentally found the tank one day in my front yard while digging holes to plant trees. That was an unpleasant surprise, cost me $10k to have the tank removed and the soil cleaned from leaks.
If it was natural gas, it would be black iron pipe, not galvanized. unscrew the cap and smell it, the smell of fuel oil never completely goes away. If water starts to leak out when you loosen it, tighten it up again right away!
This could be:
The picture is pretty clear, this pipe was installed when they poured the foundation. It does something. You are going to have to open that cap and smell and possibly stick something in there so you can see whats further in. Short term if this isn't gas you could get an angle grinder to that and shave it off at floor and fill with concrete.
Notes:
I have galvanized natural gas pipe in my 1950 Metro Detroit house. If it's not oil, it could be gas going to the original furnace location. I can see a bright spot in the floor where something stood for a while. It's common to have the furnace in the middle of the house so the heat is distributed evenly. Removing the cap and smelling is probably your best bet. But if it's gas, make sure you use pipe dope when you screw the cap back on or you will have a leak.
I would check with the city. The previous owner may have taken out a building permit for that work and the local building inspector may have blueprints of it.
If it has a 3/4 inch diameter like you say it's most likely a natural gas line. Don't cut it or gas will fill the basement!! best to turn off the gas and then remove the cap and smell for gas.. You can also have a buddy quickly turn on and off the gas while the cap is off and you can easily confirm that it's a gas line that way.
The way I can see it that looks like a 2: gal pipe with a 2: cap I think it was for a oil drum at one time for a oil furnace. Back in the day they did not use black pipe for a oil burner. Or it could be a waste line of some kind cut the pipe and check and also you can smell what it is.
I'm not sure if it is acceptable or legal to put an ohm meter to gas pipes to check for continuity. If it is, you could rig up a set of long leads and test the pipe's association to any other pipes in and around the house. You might be able to find that it is connected to other pipes that you are familiar with. Good luck.
How long gas has been available in town? Most towns, at least here, have had (natural) gas between 60s and 90s, really bigger towns had "gas factories" between second half of XIX (19th century, meaning 1850-1900) and first half of XX (20th century, meaning 1900-1950) but that gas was used mostly for lighting and cooking, so it's improbable that it would go in the basement (it was expensive and coal was a cheaper option for heat) so my guess is that leads to an oil tank or next to where an oil tank was (usually water was fed from an attic open vase, and flow/return pipes had to be bigger).
I have something just like that in my garage. It was where they filled a tank under the floor with fuel oil. In my basement there are lines that came out and went to a furnace that no longer exists.
Look around the basement for other clues. I had smaller copper lines going into and coming out of the floor. I also had a hole in the wall where the furnace vented into my chimney. The lines had a turpentine smell, which means old fuel oil.
Just to be safe, you could have the gas company look at it, they do that for free where I live. If you are going to loosen it, you could spray the seal with soapy water to check for gas escaping as you loosen.
