Take a look at Adalm Pluto or USRP B200/ B210 boards to generate GPS signal. If you want to use Pluto, you need a better clock crystal (lower ppm) for GPS usage. You will need current almanac data and the spoofing generator software on rPI. Also, if you want more fun and learn about GPS signals, you can use gnss-sdr open source software GPS/GNSS signal decoder.