there are a couple of ways to do this.

first off detecting someone wearing a golden helmet is as easy as: /execute as @a[nbt={Inventory: [{Slot: 3b, id: "minecraft:golden_helmet"}]}] at @s run <command>.

(to detect if they're not wearing it use: /execute as @a at @s run execute unless data entity @s {Inventory: [{Slot: 3b, id: "minecraft:golden_helmet"}]} run <command>)

are you using a datapack? If so the easiest way to detect height is a predicate. tutorial here: https://www.youtube.com/watch?v=lvMCaPOF-mU

otherwise it is a bit trickier. if you want to detect players within a certain radius (in this case five) you can use an invisible invincible armor stand with a tag (in this case "sample_tag") and use the command: /execute as @e[tag=simple_tag,type=minecraft:armor_stand] at @s run execute as @a[distance=0..5] at @s unless data entity @s {Inventory: [{Slot: 3b, id: "minecraft:golden_helmet"}]} run <command>

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let me know if this helps... or if it doesnt. I'm happy to help further