What is the gcc alternate of "relro" and "now" flags in "mingw-gcc"?

MinGW is actually GCC, so the flags are the same. But some flags depend on platform-specifics.

Relocation Read-Only (RELRO) is specifically for ELF binaries, which are not supported on Windows.

Instead Windows uses the PE/PE+ format (which is based on the COFF format.

There is support for -Wl,--dynamicbase and ASLR (address space layout randomization) that you could take a look at to see if it helps you accomplish your goal.