Most GPS mice for PCs are detected/recognized as a USB ACM/COM/Serial device. That means they emulate a serial port. Have a look into your device-manager to find if thats the case and which COMx port it was connected to.
You can then use the SerialPort-class (http://msdn.microsoft.com/en-us/library/system.io.ports.serialport(v=vs.110).aspx) to connect to the mouse.
A object of type SerialPort has the really nice event called "DataReceived", which is getting called when data arrives in the input buffer or (when set) when a new line of NMEA data has arrived.
Be aware that the DataReceived event gets triggered in a different thread than your GUI-thread and that you will need to invoke into your Windows-Form (or similar).