Schwinn 1979 12 speed
I think you're right on the age based on this document, 
http://thirtythree.org/projects-unconverted/Lowrider%20Bicycle%20Restoration/Identifying%20Schwinn%20Frames.pdf

G from Taiwan, in 1979.   As the document mentions, even though the date code is 79 it could have been used in 1980's line up.

The 1980 catalog shows a couple 12 speed's 
http://schwinncruisers.com/catalogs/1980.html

Only the Voyageur 11.8 says it's Schwinn-Approved making it made most likely from Taiwan.
Without any pictures or remaining decals this may be the most likely model.