How to summon a minecraft mob wearing a chestplate?
I believe the problem is that your golden chestplate is in the leggings armor slot. Here is a working command that will summon a Zombie Pigman with a Gold Chestplate:
/summon minecraft:zombie_pigman ~ ~ ~ {Invulnerable:1,ArmorItems:[{},{},{id:"minecraft:golden_chestplate",Count:1},{}]}
It is important that you list out the empty brackets inside of the "ArmorItems" tag, otherwise it will put the chestplate in the wrong spot.
I recommend mcstacker for easily making custom items, mobs, and other things as it makes it easier to not have to remember a bunch of NBT tags.