New to ROOT, hoping it has a solution to my problem. Tried to fit a curve using GSL

[Wile_E_Coyote]

I don’t really understand what you expect / miss.

I tried “GSL 2.7.1” and “GSL 1.15” and for both of them, I got almost the same output when I tried:
grep -r gsl_multifit_fsolver /usr/include/gsl

The “GSL 2.7.1” has one more function named “gsl_multifit_fsolver_driver”.

For what is available in ROOT, see, e.g.:

• ROOT Manual → Functional parts → Mathematical libraries → Minimization libraries and classes

• MathCore/MathMore → Numerical Algorithms → One-dimensional Minimization

• MathCore/MathMore → Numerical Algorithms → Multi-dimensional Minimization

• ${ROOTSYS}/tutorials/fit

• ${ROOTSYS}/tutorials/roofit and ${ROOTSYS}/tutorials/roostats

You can try the “GSLMultiFit” with the combinedFit2.C macro and/or the ${ROOTSYS}/tutorials/fit/NumericalMinimization.C macro.

UPDATE: @moneta Out of these two “tutorials” mentioned above, the combinedFit2.C macro breaks with ROOT 6.28 (it’s fine with older releases) and the ${ROOTSYS}/tutorials/fit/NumericalMinimization.C macro dies with any version (when the “GSLMultiFit” is tried).