How do you wire an outdoor timer into a GFCI outlet?

I assume you are hard wiring it and not plugging it in. The power coming into the GFCI outlet connects to the line side of the GFCI outlet. If you want the outdoor timer protected by the GFCI then connect the wire going to the timer to the load side of the outlet. If you do not want the timer protected then connect it to the line side. On the back of the GFCI if you look closely you will see Line & Load marked on the back.