This looks like a nice tool for summary stats. For confidence intervals, using bootstrap instead of student's T would be easy to implement without resorting to GSL, as well as more general.
On the subject of numerical software, here [0] is a large collection of C libraries from John Burkardt at Florida State. It includes utilities for linear algebra, stats, and other assorted numerical tasks. I've used a number of these, and have always found it clean and easy to use. [0] https://people.sc.fsu.edu/~jburkardt/c_src/c_src.html
