I'm not too clear on this but what is the 'order' of your set-up.
Breaker box to GFCI, then the light 'string', then the switch?! If this is the case, it can't see how the switch will work. Assuming my assumption (er), you should have breaker box to switch, switch to GFCI, then the light string. Hence, the switch controls the power to the GFCI.
If the latter of what I described is what you have, then the switch is bad or wired incorrectly. Post up if you have more info. (By the way, most people do use low voltage lighting but 110v. is pretty popular too. Changed to low voltage won't solve your problem.)