Have you considered writing GamoUSA or Gamo in Spain?
I am no Gamo expert, but I would try to determine if your gun had other names in the USA. For example, I bought my wife a Gamo Young Hunter at Sports Authority a couple years ago. It was also known as the Expo 26 and still appears on the GamoUSA site under archives. I don't remember exactly, but I think Expo 26 appears on the breech. I don't think Young Hunter appears anywhere except for the box. Gamo also made a very similar, if not identical, model for Daisy. I'm not sure, but I think it was the model 131.
A picture of your whole gun might help. Any chrono results might at least tell you what powerplant you have. You could also check the Chambers site to see if any similar gun is listed.
Good luck on your search.