The serial number indicates it was manufactured in the last two weeks of 1988 by Giant of Taiwan. That should make it a 1989 model but it doesn't have the small Cunningham Design decal that is typical of the 1989 version (see posts #38 & #44 for samples). Also, when Cunningham started designing the 1989 models for West Coast Cycle, he dropped the chain stay mounted U-brake as featured on your bicycle and reverted to seat stay mounted cantilever brakes. The forks were also re-designed using straight blades. Consequently, I am wondering if yours might be a foreign (non-USA) market model but I seethe pinkbike sample is located in the USA, which decreases that probability.

For reference, attached is the advertisement for the 1989 Ariel from an April 1989 magazine. It's a perfect match for the sample in post#38. If you look closely, superimposed over the last"I" of the Nishiki decal is a red bar containing the Cunningham Design logo.

It would be extremely unusual to still be manufacturing 1988 models at the end of December. This is supported by road tests and advertisements for the 1989 Nishiki models appearing in December 1988 magazines, which were typically on the shelves by late November. When you factor in all lead times, the first of the 1989 models were probably being built around September, which is fairly typical within the industry.

Barring a foreign model, the only other explanation I can suggest is that you made an inadvertent error while transcribing the serial number. Might it start with GZ7? Or might the 'Z' be a different letter?