Why does my GFCI outlet trip as soon as the breaker is turned on?
Neutrals shouldn't be connected together with the GFCI. Connect the neutral of the line and load to the appropriate connections on each side of the GFCI. Otherwise, the current will appear to go out via the hot connection and not return via the neutral on the load side of the connection, which is exactly the scenario that the GFCI detects and trips on.

Also, note this important tip from Shirlock, use the push in connectors for the GFCI outlets: When to use holes instead of side terminals to wire an outlet