To activate the OpenACC extensions for C/C++ and Fortran, the compile-time flag -fopenacc must be specified. This enables the OpenACC directive ‘#pragma acc’ in C/C++ and, in Fortran, the ‘!$acc’ sentinel in free source form and the ‘c$acc’, ‘*$acc’ and ‘!$acc’ sentinels in fixed source form. The flag also arranges for automatic linking of the OpenACC runtime library (OpenACC Runtime Library Routines).
See https://gcc.gnu.org/wiki/OpenACC for more information.
A complete description of all OpenACC directives accepted may be found in the OpenACC Application Programming Interface manual, version 2.6.