[2018] Anyone actually figure how to equip Archgun?

If I try to equip the item to the gear wheel -> you must equip a gun in vehicle screen first.
If I try to equip a gun in the vehile screen, everything says "archgun locked".
If we have to do A before B but also B before A, we are going to have problems ^^
What am I missing?

You can build the Gravimag item from your Tenno Lab in the Dojo. Once you have one completed, you can install it in the arch gun of your choice, then equip that weapon in the new inventory slot of the archwing loadout. You should then be able to summon arch guns on the ground with the gear item.