Floating pointing incrementing, not working

Projects Programming
10 
    totalGallonsA =+ flowG ;

That isn't the same as:

    totalGallonsA += flowG ;

Your version merely sets totalGallonsA to the value in flowG. It doesn't add.

    // Disable the interrupt while calculating flow rate and sending the value to

// the host
    detachInterrupt(sensorInterrupt);

Better is:

noInterrupts();

    // Enable the interrupt again now that we've finished sending output

attachInterrupt(sensorInterrupt, pulseCounter, FALLING);

Better is:

interrupts ();