I'm using Qt 5.8 and gsl 2.1 (compiled with cygwin64), this is what i use in the .pro file to link gsl:

INCLUDEPATH+=C:/gsl/include/
INCLUDEPATH += C:/gsl/LIBS
LIBS += -LC:/gsl/lib -lgsl -lgslcblas -lm

when i include the headers and compile i don't have any problem even when i declare and object, but when i tray to use any function i got this error:

C:\Users\CIFAS-1\untitled4\mainwindow.cpp:9: error: undefined reference to `gsl_matrix_alloc'.

Please any one has some idea?