unable to assemble using ml/ml64 - gcc

Question

I've been using Visual Studio to assemble and link cpp programs that include masm (.asm) files. I want to be able to do this myself, on the command line. I used the ml.exe that Visual Studio provides to assemble my masm code (at Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\bin\Hostx86\x86\ml.exe).
However, when I try to compile my project with gcc main.cpp fib.obj -o main, I get an error saying that the .obj architecture does not match the target architecture (i386 architecture of input file 'fib.obj' is incompatible with i386:x86-64 output).
So my question is, simply, how do I compile this project?
What I've Tried
The two things that I've done to get different errors (which I think are more promising) are (1) to utilize the ml64 executable from ...\Hostx86\x64\ml64.exe or ...\Hostx64\x64\ml64.exe, and (2) to use the -m32 option for gcc.
ml64
Using ml64 I have been unable to assemble my masm code. with the following source file, ml64 spits out this response:
; int fib(int);
.386
.model flat, c
.code
fib proc uses ebx ecx, i:dword
mov eax, 0 ; current fib number
mov ebx, 1 ; next fib number
while_1:
cmp i, 0
jle while_1_end
; add eax -> ebx and mov previous ebx into eax
mov ecx, ebx ; store new current fib number
add ebx, eax ; sum previous fib numbers
mov eax, ecx ; set eax to current fib value
dec i
jmp while_1
while_1_end:
; the ith fib number is in eax
ret
fib endp
end
...\fib> ml64 fib.asm
Microsoft (R) Macro Assembler (x64) Version 14.29.30136.0
Copyright (C) Microsoft Corporation. All rights reserved.
Assembling: fib.asm
fib.asm(2) : error A2008:syntax error : .
fib.asm(3) : error A2008:syntax error : .
fib.asm(6) : error A2008:syntax error : ebx
fib.asm(24) : fatal error A1010:unmatched block nesting : fib
I presume that .386 isn't right for a x64 target, but removing it changes the error very little (it only removes the syntax error line 2).
I don't really want to use x64 assembly if it isn't the same as x32, but I'll use this solution if it works. I think my second option is more promising.
gcc -m32
I don't believe I can tell gcc to build to a 32-bit target because I don't have 32-bit versions of the libraries. When running gcc -m32 main.cpp fib.obj I get cannot find -lmingw32, cannot find -lgcc, cannot find -lgcc_ex, etc. Should I get these libraries, and if so, where do I find them? It seems a bit awful to download 32-bit libraries on my x64 computer just to assemble masm, and I don't think I should have to, seeing as Visual Studio is able to compile my project simply by setting the target to Win32.
Is there a different way of telling gcc to target 32-bit architecture, or are the libraries perhaps hidden in the Visual Studio files, or something else? I'm pretty sure that if I could get gcc to target x32, it would solve my problem, because Visual Studio targets x32, and when I change the target to x64 I get the same error I have been getting (module machine type 'x86' conflicts with target machine type 'x64').