Scrub the iron gate's lock latch with a wire brush to knock off as much rust, paint and dirt as possible.
Plug a right angle grinder into a nearby electrical outlet or into an extension cord.
Hold the right angle grinder's round blade perpendicular to the iron gate latch and turn it on. Cut between the latch and the gate slowly to avoid the latch flying off and striking someone or something. Wait for 10 to 15 minutes for the gate to cool.
File the area with a metal file to smooth it out and finish.