"You are starting inputs from the web interface now. Go to System -> Nodes -> [Action dropdown] -> Manage inputs and start as many GELF inputs as you want." - Lennart Koopmann (@_lennart)
For those of you who are interested, my question was answered by Lennart Koopmann (@_lennart) on the Graylog2 google group: https://groups.google.com/d/msg/graylog2/uEyU0IA-O-4/Ni-Zx9NShL8J