It shouldn't matter exactly how you have your development environment set up. Most assemblers use very similar syntax.
I found this documentation on mixing assembly with your C code however I couldn't find any documentation on which assembly syntax they use so you'll have to experiment I guess. I also can't see any support for macros so you might have to write a specific function for each debug message you want.

You'll want to write something like this in your .s (assembly) file for each debug message you want:

.global my_msg
    ld  d, d
    ret
    .db 64, 64, 00, 00
    .db "hello world"

As I said, I'm not sure on exactly which compiler directives it has so research how to write data like I have with the db directive.

__asm blocks can accomplish this in C:

#define DEBUG(message) \
__asm \
    ld d, d \
    jr .end \
    .dw 0x6464 \
    .dw 0x0000 \
    .ascii message \
.end: \
__endasm

For non-constant strings, they must have a fixed address and bank:

#define DYN_DEBUG(address, bank) \
__asm \
    ld d, d \
    jr .end \
    .dw 0x6464 \
    .dw 0x0001 \
    .dw address \
    .dw bank \
.end: \
__endasm

I tried this:

#include <gb/gb.h>
#include <gb/drawing.h>

// this allows you to debug in BGB
#define DEBUG(message)
__asm \
    ld d, d \
    jr .end \
    .dw 0x6464 \
    .dw 0x0000 \
    .ascii message \
.end: \
__endasm

// do dynamic debugging with a bank
#define DYN_DEBUG(addr, bank) \
__asm \
    ld d, d \
    jr .end
    .dw 0x6464 \
    .dw 0x0001 \
    .dw address \
    .dw bank \
.end: \
__endasm

void main () {
    gotogxy(7, 8);
    gprintf("O HAI!");
    DEBUG("O HAI");
}

and go this error:

demo.c:6: syntax error: token -> '__endasm' ; column 13
make: *** [<builtin>: demo.o] Error 1

I am using gbdk-2020.

@konsumer If you're using gbdk-2020 then you can just #include <gb/bgb_emu.h> and use BGB_MESSAGE.

However, I did forget a couple backslashes. That should be fixed.

That works great. Thanks so much!!! I didn't realize they have their own header for this. works perfectly.

#include <gb/gb.h>
#include <gb/drawing.h>
#include <gb/bgb_emu.h>

void main () {
    gotogxy(7, 8);
    gprintf("O HAI!");
    BGB_MESSAGE("O HAI!");
}