What if you don't have the source file (sumFirst.c) but do have the executable file sumFirst or the object file sumFirst.o?

The sumFirst.o file of machine code bytes is generated from sumFirst.s by the assembler that is called by gcc.

In this case you can't use gcc with the -S option since that would require having sumFirst.c.

The general utility od lets you examine the bytes of any file whether they contain printable characters or not.

The od name is short for octal dump, meaning that each byte of a file is displayed in base 8 notation by default. However, you can change that to hex or other formats.