As Dan in CA said, there is a plugin for the interval program to directly turn GPIO pins on and off.
I have a small collection of relay boards, active high and low and they all work with the RasPi. Some are better than others, see the link for more info. I have driven a relay board with both ospi.py and sprinkler_pi.
You say you tested GPIOs with an LED, I hope you used a current limiting resistor. The RasPi GPIOs are somewhat fragile and easy to burn out. The opto-isolated relay boards I have used have all worked fine.