Reading usb data from external device using Arduino Zero

Since the GPS board has an RS232 output I wonder if it is possible to have a second serial on Arduino Zero and use it instead.

Yes it's possible.

Here's a tutorial by Adafruit on how to add extra hardware serial ports to your Arduino Zero: Creating a new Serial | Using ATSAMD21 SERCOM for more SPI, I2C and Serial ports | Adafruit Learning System.