Gutierrez is a actually a good driver, he was just too young for GP2 and F1 when he got there (ART and Sauber were both off-form when he was there too), I think the year at Ferrari will have helped him a lot like it did for Massa in 2003.
Well Max Verstappen is only seventeen and has shown not only great speed, but great race craft as well. During his two years with Sauber Gutierrez showed neither of these two attributes. Even Massa in 2002 as erratic as he was, showed he had the pace. In the end age is no excuse. You've either got it, or you haven't.