Merge branch '100-gsl-build-version-2-6-gcc-10-2' into 'master'

Resolve "gsl: build version 2.6 GCC 10.2"

Closes #100

See merge request Pmodules/buildblocks!135

Compiler/gsl/files/variants.rhel6

@@ -14,3 +14,4 @@ gsl/2.5		stable		intel/19.4
 
 gsl/2.6		stable		gcc/{7.4.0,8.3.0,9.1.0,9.2.0}
 gsl/2.6		stable		gcc/{7.5.0,8.4.0,9.3.0,10.1.0}
+gsl/2.6		unstable	gcc/10.2.0