The RX0 and TX1 pins are a second serial port available as Serial1 so you should be reading GPS data from Serial1 and printing it to Serial (the Serial monitor)
Your sketch checks whether data is available on Serial and if so, reads from Serial1 and writes it to Serial. It should check whether data is available on Serial1 then read it and write it to Serial
You mean the code that you changed after I pointed out that it was wrong ? Don't change posts, particularly code, that has been commented on as it makes nonsense of the comments.
If you change code then post the new version in a new post. Do that now. Post the code as it is now and add comments explaining what each line of loop() does or should do
Serial.begin(9600); //Serial monitor baud rate set at 9600
Serial1.begin(9600); //GPS baud rate set at 9600
}
void loop() {
if (Serial.available()) { //Checks whether data is available in Serial
Serial1.write(Serial.read()); //Then Reads in Serial and writes in Serial1
}
if (Serial1.available()) { //Checks whether data is available in Serial1
Serial.write(Serial1.read()); //Then reads in Serial1 and writes in Serial
}