Is my outbuilding wired correctly?
You should have only attached the white grounded (neutral) conductor from the cable leading to the other receptacles, to the grounded (neutral) terminal of the GFCI.  The white grounded (neutral) conductors for the lights should be connected with at pigtail to the neutral feeding the building.



Unless you want to provide GFCI protection to the security light as well, then you'll connect the wires feeding that to the load side of the GFCI.