Does gcode require a response code from the interpreter?
See: http://en.wikipedia.org/wiki/G-code
G-code is a one way protocol; I think echoing of interpreted commands is very common.  Otherwise the DNC terminal would be blank.

Traditionally, the target machine will halt and display and alarm to the operator (a person) if an error occurs.

I doesn't matter if the interpreter has processed the command.  What does matter is if the communications buffer has room for more data.

The machine will use Hardware or software flow control of the communications link to prevent the sending computer from overflowing the buffer.  The specific implementation is related to what kind of link you are using.

See: http://en.wikipedia.org/wiki/Flow_control

If you plan on using something fancier than a terminal program, you can send what ever you want back.  I don't know if there are any standards.  Don't confuse it with systems that do the interpolation on the PC, such as Mach 3 (http://www.machsupport.com/).