Specifically, this is for 32 bit, .intel_syntax noprefix, GCC "C" asm output obtained with -masm=intel -S options.
The -S option produces the asm file (usually passed on to as).
Assemble with: Todo: section and ???
Added align to gas.inc and included align.inc and org on command line for linking flat binary.

Very interesting idea!

Does GCC produce register-based addresses with a strictly predictable structure? If it is always like displacement[base+index*scale] (plus simpler variants) then you could in theory replace the algebraic address parser that standard fasmg's macros use with a dedicated one. And parsing the AT&T syntax, as horrible as it is for anyone used to Intel one, could be probably even simpler.

I was thinking more along the lines of having only one "C" compiler and letting fasmg generate multiple formats
instead of a native compiler and many cross compilers.

I think there are not much people who like the AT&T syntax at all.