I'll also recommend the same things that you have already indicated. Do sand the steel before painting it. If it has any rust or scale on it this must be removed by vigorous wire brushing till all the rust is gone. Spray painting is really the best way to paint steel so you must consider appropriate masking of adjacent areas so over spray does not get on other parts of the fence or driveway.
The best paint for this job will be an enamel paint specifically designed for the job of painting on metal. For a relatively small job, like this one seems to be, I would simply recommend that you use the spray can type that you can purchase at the home store or hardware store. Do use multiple light coats of paint to get a nice finish without problems with the paint running or dripping. The only drawback with spray cans is the limited range of colors available.
One last thing you may want to consider is the possibility to temporarily remove the steel cross beam from the gate and paint it separately so that you can get it painted from all sides. Then re-install it and go back and touch up the fasteners with a spritz of the spray paint.