Thanks for your project. The contents of the GSL are
Complex Numbers
Roots of Polynomials
Special Functions
Vectors and Matrices
Permutations
Sorting
BLAS Support
Linear Algebra
Eigensystems
Fast Fourier Transforms
Quadrature
Random Numbers
Quasi-Random Sequences
Random Distributions
Statistics
Histograms
N-Tuples
Monte Carlo Integration
Simulated Annealing
Differential Equations
Interpolation
Numerical Differentiation
Chebyshev Approximation
Series Acceleration
Discrete Hankel Transforms
Root-Finding
Minimization
Least-Squares Fitting
Physical Constants
IEEE Floating-Point
Discrete Wavelet Transforms
Basis splines
Running Statistics
Sparse Matrices and Linear Algebra
Could GSL users here say which categories have a lot of functionality that is missing from existing Fortran libraries. Knowing that would help a Fortran programmer know when to call GSL and also suggest areas where open-source Fortran developers could fill gaps.