Making an OpenSource GPS is technically challenging. For an idea of what is involved see: http://www.aholme.co.uk/GPS/Main.htm

Here are some other similar projects: http://sdrgps.blogspot.com/ https://github.com/hahnpv/gr-gps https://github.com/hahnpv/SoftGNSS

Personally I would rather buy a $10 modules off eBay or a Chinese website which provides the GPS data in a standard NMEA serial stream.