Assess the extent of the rust on the gates and prepare accordingly. The more rust you have to remove, the more brushes and grinding discs you will need.
Brush away the paint and rusted areas on the gate using a stiff wire brush. Rub the brush back and forth to remove the rust. Make several passes until all rust is removed from an area before moving on to the next. Use a grinder and abrasive metal grinding disc to remove paint on the larger areas of the gate like the vertical slats, but be careful not to grind away too much metal. Use a feather-light touch on thin slats, for instance.
Clean up the remaining rust using smaller abrasive pads or steel wool. Rub the pads into the crevices and corners of the gate where necessary. Then clean the surface of the gate with a towel to remove all rust residue.
Prime the iron gate with an alkyd-base primer paint. Spray the primer on the gate soon if not immediately after removing the rust. Allow the primer to dry.
Apply a coat of exterior metal paint using a spray can for a smoother finish. Let the coat dry and apply a second coat if desired. Be sure not to miss any hard-to-reach spots, or rust will occur.