Crap or not?
Might depend on what you want it for and expect it to do. Do I assume you are looking at springers? Are they universally crap? In my opinion, no, but do not expect to go out onto an HFT course and beat all the top end springers that most likely cost 2, 3, or 4 times as much as your Gamo. Are they a good rifle for a bit of fun / plinking/ short range ratting? I guess so, if you do your bit. Will the build quality equal that of a £300 - £500 rifle and have similar accuracy? No, do not expect this but what you could get will be a pretty reliable mechanism that is great fun and a good introduction to air rifle shooting.
The advice about buying 2nd hand is good ... but only if you know what you are doing as it is too easy to buy a rifle with unseen faults that an experienced shooter may well notice. For instance I have recently seen a cosmetically excellent good quality rifle that to all intents and purposes was a dream. But over the chrono it was only putting out about half the specified power. The fix was pretty easy, but maybe not if you have no knowledge of what to do or, the inclination to do it. And of course, parts must be factored into the equation and they are not the cheapest items ... maybe £20 for a mainspring, £8 for a piston seal and £5 for a breech seal (all most likely including postage). Buying from an RFD may, or should, remove these worries, but not always.
Sometimes buying new is worth it for peace of mind.
Cheers, Phil