Thxs. It's no earlier than 1988 based on the components, but it has the 1989 decal scheme. I'm going to list it in my database as a 1989, with a questionmark.

If you look at Tommy's known 1988 Backroads (thxs for confirming Roccobike!) with GZ7xxxxx serial number and yours being GC9xxxxx, my instinct is that the format is G= Giant, C & Z are fortnight indicators and 7 & 9 are calendar years. Of course this is a very limited sample size, so we're going to need quite a bit more data to build our confidence, but now we have s/n model as a starting point for testing any future data.