SandyC:  Yea, Im sure.  My wife and I have been going to GC in December for 10 years.  Obviously temps vary year to year (and I dont go out of my way to see what temp it really is) but mid 70's with a stiff breeze is a pretty good guess at night.  Will you need a sweater?  Who knows, but we like one, especially if we are laying out in a hammock at night.