I spotted the sale. I'm tempted but will probably hold off - money's a little tight now for luxuries.
As far as the age...
Since they're Traditional and not Alpha, they're at least as old as 1999. While it's true that barcodes didn't start appearing on decks of playing cards until the early-mid 1970s, it's also true that even today some decks are produced with no barcode at all - I have a number of Bicycle decks, all current or close enough to it, that have no barcode on the bottom, and they're not all commissioned custom job.
But assuming that Gemaco did as USPC and issued barcodes at about the same time, I'd say that these cards were made between 1965 (because they don't have a tax stamp sealing the deck shut) and about 1975 or 1976.
However...
Sticker-type deck seals, at least from USPC, weren't being used until sometime in the early 1980s, if my memory is correct. This deck has a sticker seal, not a stamp seal. This would put your decks at being somewhere between 13 and about 30 years old. But this does assume again that Gemaco was operating on the same time table as the USPC with the introduction of sticker seals.
Do your open decks mention a copyright date anywhere?
@Dazzleguts, don't expect too much, most reviewers would say "I still like USPC cards more" in their conclusions, and he is one of them. Gemaco decks have 2 series: traditional (old) and alpha (new, since 1999), and this deck belonged to the traidtional series. He thought the cards are cheap ($2-3), stiff, easy to spread and faro, but the back design is not good for magic and flourish, can only be used in casinos or for gambling demo (but he preferred using Bee in this application ).
Back then, Gemacos would have been of better quality than they are today. And as of just a few years ago, Gemaco is yet-another subsidiary of USPC, itself a subsidiary of Jarden. Whether or not quality has improved as a result remains to be seen, since I haven't yet seen a Gemaco deck made under USPC supervision (which took place in 2000, just after the introduction of the Alpha decks).