GPA alone tells you about intelligence in picking classes/work ethic and little else. It is the course load that may give clues on capabilities and aptitude. If someone majored in mathematics, that needs to be recognized as a more rigorous major than a major in women's and gender's studies. The GPA might say that the latter (e.g. 4.0) is "smarter" or "works harder" than the 3.4 math major, although that may not be true at all.
MCAT is more arbitrary (just 1 day) than GPA, but since the "curriculum" is set, it can't be "fixed" via "course-fixing" (easy and fake classes to inflate). It is also standardized and curved based on other test takers. Thus, it is a more uniform and reliable measure of raw aptitude than GPA, but does not tell much about long term work ethic. The bright side of the MCAT (from my biased perspective), is that it gives people with a poorer work ethic/harder major/poor pre-med "management skills" a chance to counterbalance the GPA.