I'm gonna guess it's an '88 model, built by Giant for Schwinn. Based on the "G" in the serial # and I believe the third digit is the last digit of the year. If there are four small numbers stamped on the headbadge, that's the date code. First three digits are the numerical day of the year, the fourth digit is the last number in the year. Example: 1868 would indicate the bike was assembled on the 186th day of 1988.