This is the fitting connecting the copper (yes, copper tubing is OK for gas lines where I am) gas line feeding our water heater to our water heater. What size and type of fitting is it?
Two types of flare fitting, the 45° and the 37°. The 45 is used on low pressure air/gas/water and also rated for fuel gas (usually LP, aka propane). The 37° is called the JIC flare and used in hydraulic plumbing. They don't interchange so just be aware when you buy tooling and fittings.
Can't tell from the picture but I'm guessing 1/2" hard copper tube to 1/2" 45° flare to NPT.
Get a caliper or something to measure the tube. If the OD is exactly 0.5" then you have soft copper. If it's 5/8 then you are looking at hard copper (the kind typically used for water) which for some reason doesn't go by outside diameter but a nominal inside diameter trade size. If it's different, refer to the handbook linked below to figure out the size.
Hard copper can't be flared without first annealing it. To anneal, I heat with a propane or mapp torch until it is glowing orange hot and the flame is turning green. Then let it slowly air cool to anneal. Quenching it with water will only harden it again. You can then flare it in a flaring tool.
Plenty of information in this handbook if you wish to read further: Copper tube handbook
If I had to guess, that's a 1/2" NPT [to] 3/8" compression elbow. It's possibly a flare fitting instead of a compression. Which wouldn't really matter; as long as you get the right size for the copper, it will come with the right escussion (or at least you'll know which to get - assuming you're not required by your codes to use flare fittings for gas).