Good looking set, I have a lot of experience with old gon bops. From the pics they appear to be in pretty good shape, the hardware especially. I notice a couple of splits in the staves, which is very common in gon bops as the glue they used was not the best quality. They can be easily repaired though. The sizes you have are nice, gon bops ran smaller than what is considered standard sizes today, your set is what is considered more standard these days. Their quintos were 9.5 and congas were considered 10.75, 11, and 11.25, everything else they considered a tumba. It is much more common these days to find the smaller sizes, so yours may bring a little extra based on that.
The international models were made with red oak which is very heavy compared to their mahogany models, some players prefer the mahogany due to the light weight. In my opinion the oak is much more solid and produces a better sound. The hardware on these models were top notch, heavy duty and have the reinforced sideplates, you also have the more desireable traditional style rims.
Based on the pics, I'd say they would pull in around $250-$300 per drum, maybe a $100 more for the 13.25 as the larger tumbas are quite rare. In my experience with buying and selling these drums I have noticed that they are easier to sell seperate and will bring you more money that way as well. They are very expensive to ship, and when you are shipping 3 of them that tacks on $150-$200 to the buyers, therefore they aren't willing to pay as much for them.
Thats a great looking set of drums, the Gon Bops oak internationals are by far my favorite congas ever made, and I've owned lots and lots of them. If I had some cash set aside I would make you an offer for sure.
Here is a pic of my Internationals, one of very few oak sets I've seen with out the inlayed band:
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