I think you should write something like (I have wrote assembly for years):
CalcGdcI PROC
xor edx, edx
mov eax, DWORD ptr[esp+8] ;dividend
mov ebx, DWORD ptr[esp+4] ;divisor
loop:
div ebx ;eax/ebx
cmp edx, 0 ;remainder in edx
je L1 ;yes: quit
mov eax, ebx ;restart with eax = ebx
mov ebx, edx ;and ebx = edx
jmp loop
l1:
...