The standard serial port on the Magni us used to talk to the MCB board.
The signals on P508 and P507 are not available for usage as they are needed for Magni operation.
What I often do that works out very well for adding serial devices is get a usb to serial port device, often called a FDDI port. These come out with a connector that will supply your GPS with grond, 5V (some have 3.3V) and a receive and transmit. A great many GPS devices can be used with this style of device. Use these keywords to find them ‘usb to serial cp2102’ You can find them very low cost on EBAY or can find them at popular hobby electronics sites like sparkfun or likely at Adafruit. I get them on Ebay. Take a look at these two possible ones, I prefer the once based on the Cp2102 type chip. Look at these https://tinyurl.com/yb9jqvs4 and https://tinyurl.com/y8c7d2z4