Will this header work with my stock exhaust manifold gasket?

You'll need to verify the bolt pattern matches your engine. Bring your OEM gasket to the install and test-fit before committing. Most drivers grab a quality header gasket kit to seal properly.

Do I need to tune or reflash the engine after installing this?

A header alone won't require a tune—it bolts to the same mounting points as stock. If you're adding other modifications (intake, exhaust, cam work), then tuning becomes important.

Can I paint this or does it need any prep?

The polished stainless finish is ready to install as-is. If you want to paint it, light scuffing with fine grit and a high-temp header paint will stick. Most owners leave the stainless exposed because it doesn't corrode and looks clean in the engine bay.