Solid 18k, or even 14k, would add significant value to the watch. 9k would be worth less, and plating much less. I suspect that it is solid, as it has an "integral" bracelet, likely made of the same material.
Another complication is that it is not very desirable in the collectors market because of the style. That's not to say that there wouldn't be any interest, but it would be very low, relative to the many, many more desirable models on the market.
For that reason, the gold content is, sadly, the most important variable. You could discover the content by having a watchmaker remove the back of the case, as it will be stamped inside.