gcc -### - show what GCC would have executed
gcc -v - show what GCC is executing
gcc -g x.c -o x; objdump -S x - show the C and generated assembly code
gcc -E -dM - gcc -C -E - show pre-processor output, but leave comments intact
gcc -M - show all include file dependencies (for use in Makefiles)
gcc -MM - like above, but ignore system include files

via LWN.