According to Harvard University, a 3.4 GPA on a 4.0-point scale is somewhere between a B+ and an A-. Stanford University calculates GPA equivalency in letter grades similarly, with an A- being 3.7 and a B+ being 3.3.
According to both Harvard and Stanford, GPA is calculated by taking the average of all letter-graded courses. Stanford's official website clarifies that grades and symbols other than the standard letter grades (A+ to D- and F or No Pass grades) are not utilized in calculations of GPA. Stanford also makes clear that the weight of a particular letter grade depends on the unit value of the relevant course.