Gero being the foundation of your argument is problematic for two main reasons:
A) The inner workings of his absorption method are kind of a grey area, because although he absorbed ki from some of the Z Fighters, it's channelled through his mechanical energy source. So we're dealing with an energy system with undefined boundaries (excluding our knowledge that it's not an infinite energy type), and a situation that never even came into actuality (i.e. Gero not getting the chance to absorb all of them and for us to judge the resulting power-up).
B) His calculations were often proven wrong, both due to his overconfidence and lack of awareness. With that in mind, why does the 50x boost have to be what's at fault, and not Gero himself? There doesn't seem to be any merit in targeting Super Saiyan itself, while Gero saying something questionable or inaccurate falls in-line with how his character was portrayed on a regular basis.
It's also worth noting that although Gero turned out to be inferior to Piccolo and Vegeta, the extent of the difference isn't set in stone. So throwing around the term 'impossible' as if you have the blueprint to Gero's scaling seems far too assumptive.