if (Serial.available()>0)
{
incoming = Serial.read();
switch (incoming)
{
case 71: // "G"
The value 71 is NOT the string "G". It is the letter 'G'. But, instead of a comment that says that 71 is G, why not just make the case involve the letter G?
if (Serial.available()>0)
{
incoming = Serial.read();
switch (incoming)
{
case 'G': // No comment needed to describe what to send...
I am trying hard to write but cannot.
Not to belittle your efforts, but your code does not reflect that.
Robin2 has a great tutorial dealing with serial input basics
Separate the reading of serial data from using that data.