I suspect your "GIG" is actually what Derek said - "C II G" (being CG for Caithess Glass with the Roman numerals "II" between indicating "2nd"). But with the letters close together it can sometimes look different, particularly when it is scratched on rather than etched.
Caithness Glass had a policy of not destroying weights that had a flaw of some sort but were still of a quality that would be acceptable at a cheaper price than for a perfect example. They sold those 'second quality' items through their factory shop outlets.
In some cases, such as with abstract designs, the flaw can be hard to see unless the weight is compared to a perfect one. In other instances, the design may be, say, off-centred and be a more obvious fault but not so much that would cause it to be destroyed.
Caithness were happy to at least make some money from such weights and people who could not normally afford the perfect examples were happy because they could have a weight that was, in most respects, very reasonable to look at.