Without welding on hardware? 12' panel needs to hang on a 4 by 4.

Screw 2 gate hanger bolts into the well-braced 4x4. Pipe fence hinges and probably a floating wheel at the other end as 12' is a lot of pull on a post.

Tractor Supply has the pipe gate hinges. They just clamp with bolts onto the panel wherever you want them.
As Wood said use lag bolt type screw in hinge pins that will fit your pipe clamps hinges to screw into your 4X4. You need to brace your 4x4 or the gate will sag. If your panels have pipes for the pins you can just space your hinge pins to match those if you have room on your 4x4.

Thanks guys. 4x4 is part of the pole barn and not free standing. Do you still think I'll need additional support?

Potentially not then, but if you want the panel to stay at an ideal height you might consider putting a wheel on the bottom of the panel leg as suggested in the 1st response. This will assure less sagging/ stress and make it a lot easier to both handle and not blow around with any wind. And if you hang it low enough to prevent a large animal from going underneath, that leg is gonna be near the ground anyway.

They sell those wheels at tractor supply, too.