Fortran compilers (gcc has a Fortran 95 frontend) have long offered optimization levels beyond -O3; i.e., -O6 is a decent upper bound among popular free and commercial compilers, not just gcc/gfortran. The author probably came from a HPC background where -O6 is commonly used for compatibility among such a variety of compilers, even when writing C and C++ programs. Technically not the most portable way to build, but it works.