I've seen a few of 'em that shot well, but I had a Whisper Deluxe a couple of years ago, it had an expert tune, and it was never accurate, after all manner of trouble, several scopes and mounts, I swapped it away for some tile work. As I looked critically at it, trying to get a solid mount, and trying to reduce the thump of the hollow stock, I came to see it as cheaply constructed. I "replaced" the Whisper with a Slavia CZ-634 (about the same price) and it has been a great gun, never any problem (thanks for the tune job, Gene).
My next door neighbor's son bought a "Big Cat" and he and I have had a few sessions trying to get it shooting right. I gave him a new BSA scope and one piece Accushot mount to replace the POS that Gamo calls a scope, and John (who is quite expert at guns in general) spent hours trying to get it zeroed. Meanwhile, I would put ten pellets in a dime spot at 25 yards with my Czech. He has tried various pellets, and I believe the scope is properly sighted, but his best patterns are easily 3X the diameter of mine, not counting flyers. I think it is still dieseling, so maybe it should really be torn down and tuned. But I think you have to buy several Gamos to find one that is accurate. I considered ordering a GRTIII trigger (after the last session last week, I laughed as he told me his trgger finger was sore after maybe 100 shots). This gun has the metal trigger group, but I am of the opinion that further efforts are dubious, he ought to remove the scope, use it for killing grackles at short range, and get a better gun.
With the change to use their own plastic trigger groups that can't be upgraded (yet), my advice is stay away.