What's the best way to paint a exterior steel door with gloss paint. I

https://www.thisoldhouse.com/ideas/paint-high-gloss-front-door

After cleaning the door, use spray car paint, making sure that you protect surrounding areas and the hinges. Several light coats are better, allowing plenty of drying time in between. Good luck - Sue

Buy spray paint for metal surfaces

I would clean it up real good, sand it down to take care of any rough or rusty spots and give the paint something to adhere to. Use a paint made for metal, like Rustoleum in high gloss. It would probably be easiest to paint if you take it down and put it on something. I would also use spray paint, with light multiple coats you will have a smooth finish with no drips or brush strokes.

Rustoleum works very well on ferrous metals