There is two possibilities here:
- Either you just want to be able to use C99 features; in this case you don’t have to do anything, as the default selection for GCC is -std=gnu11, that is C11 plus GNU extensions, so it supports C99.
- Either you want to be sure you are fully C99 conformant to standard iso9899:1999, with no GNU extensions, then you should effectively select -std=c99, but you should also add -Wpedantic, as otherwise it will still accept some GNU extensions, as well as some c11 features...
BTW, the default for C is -std=gnu11 and for C++ it’s -std=gnu98.
Hope this helps,