There is no GA5...it must have been a typo.
The listing looked like this:
GA4 Light Gunmetal Metallic
GA5 Dark Gunmetal Metallic
So, while it may well be wrong, it definitely wasn't a typo.
The letter stands for the year the color was introduced. F=1970 G=71 H=72 J=73 K=74
This I did not know. Thanks.