Armor Stand can't have an end gateway block on their head(it won't be displayed)

What exact commands are you trying to use?

I'm using the data command. But I also tried using the summon command. I have no shaders.

The exact command:

/data merge entity @e[type=armor_stand,limit=1,sort=nearest] {ArmorItems:[{},{},{},{id:end_gateway,Count:1}]}

this works with any other block but not the end gateway

Only items can be put in the armor slot. End gateway doesn't have an item form.