thanks for your message.
In case you installed GSL as suggested on the webpage in its own /gsl directory which is located under the standard search path of your compiler, a simple #include <gsl/… .h> should be enough.
For more information, please have a look at the GSL user guide (Section 2.2 Compiling and Linking).
If you have any further questions, please let me know.
PS: Further below in the GSL user guide (Section 2.2.1 Linking programs with the library), a basic gcc compiliation command is listed with additional compiler flags. You might want to include these flags in the specific Makefile in your OpenLB application. The section in the Makefile typically reads
PPS: GSL typically requires an additional algebra subroutine library. Hence the final compiler command (if the library is linked to the default path) in your Makefile should end with ... -lgsl - lgslcblas -lm -lz -o $@.