Create a 'bypass' on the serial port for Arduino.

ArduinoSerialEchoTest.ino

//this can be used to test a uart connected GPS.

void setup() {
  // initialize both serial ports:
  Serial.begin(9600);
  Serial1.begin(9600);
}

void loop() {
  // read from port 1, send to port 0:
  if (Serial1.available()) {
    int inByte = Serial1.read();
    Serial.write(inByte); 
  }
  
  // read from port 0, send to port 1:
  if (Serial.available()) {
    int inByte = Serial.read();
    Serial1.write(inByte); 
  }
}