AFAIK, Gecado airguns were made by Diana. I suppose pistols as well as rifles.
If it is similar to my Diana 5G, you have a mighty fine pistol there.Mine was bought in the earlier 1980's. After 25 years+, the piston seal was falling apart.I believe it has the GISS system, do a google search.This is not easy to work on.Enjoy,-Y
CONGRATS!Looks a lot like my Diana Model 5 (Winchester model 353)I think Diana sold them under several brandsMaybe this will help.https://www.pyramydair.com/blog/2017/03/dianas-model-5-air-pistol-part-1/https://www.pyramydair.com/blog/2017/03/dianas-model-5-air-pistol-part-2/https://www.pyramydair.com/blog/2017/03/dianas-model-5-air-pistol-part-3/
Try putting some penetrating oil (Kroil) down the barrel, letting it soak a day or two, then tapping from the muzzle towards the breech end?(P.S. WD-40 is not a penetrating oil)
Probably not a good idea to use BBs they are steel. I cringe to think of the impact on the bore. I suggest getting a can of cheap lead pellets at Walmart and use them to see if things are working better. While not the best idea for a gun in excellent condition, for one this abused, get a pistol cleaning kit and .177 brass or bronze brushes intended for powder firearms. Hoppes and others make a solvent designed for removing lead from powder guns. Use in place of the secret sauce. Fill the bore with it and let it sit for a few days. Then scrub the bore clean and dry with patches. Hopefully, the rifling will survive the treatment. As noted, dissolved lead ingested is a really really bad idea so don't lick your fingers!!! JOKE!! Wear plastic or rubber gloves when working with it.