Rename “main(int argc,char const *argc[])” into “test(void)”, then try:
root [0] gSystem->Load("/usr/lib/libgsl.so"); // see: locate libgsl.so
root [1] .x test.c++
and / or:
root [0] gSystem->AddLinkedLibs("-lgslcblas -lgsl"); // see: gsl-config --libs
root [1] .x test.c++