Gonzo follows a pattern of quantity over quality. They produce more shows in one season than some studios produce in a year. As such, their individual quality varies wildly and tends towards the mediocre at best.
They hire good conceptual people (character & mechanical designers) but not people great at implementing the concepts (directors, writers, animation directors) and subcontract animation to second-rate studios, which produces sub par results.
It should be noted, they're not well regarded in Japan either. Though they produce a lot of shows, barely any have been successful in the last few years and the financial strain of that has just about bankrupted their parent company.
That said, once in a while they produce a gem and sometimes produce some shows that while not amazing are at least good and entertaining. But they're too few and far between for the studio as a whole to maintain a good reputation.