I've never actually used gcc to target UEFI, but under Clang and other LLVM based compilers it really is just a few flags and header files. If I recall correctly my little demo linked above predates any sort of UEFI support in zig and just uses what LLVM provides, you can see how straightforward the whole thing is in the makefile (the little bit of assembly is unrelated to UEFI): https://github.com/liampwll/zig-efi-os/blob/master/Makefile

Edit: Here's a minimal example using clang from someone else: https://github.com/AndreVallestero/minimal-efi