So, this must be a common problem. Brass nut pulled too tight and coming loose in plastic gas tank, when tightening the petcock down, plus a crack in tank, next to it.
Tank is made of ABS blow mold plastic, and patching material might not stick to it.
You can plastic weld it with a good soldering gun that has a wide flat tip, using great care. I have both a plastic welder that I got from Harbor Freight & plastic welding rod. There was a problem with what they say was ABS plastic rod, as it didn't take well. I ended up doing the repair with my soldering gun as I described. In the process, you drag melting plastic into the crack and back over and around the brass nut, being careful not to make the nut looser. The melted plastic all welds back together solid.