Great project idea, however, I'd simply make use of a cheap Arduino (ebay) and the GPS module from Adafruit for $40. Then drive BCD out of the Arduino pins to your Nixie tubes. You'll only need 4 pins per tube for the BCD control.
As we unveil EE Times’ 2015 Silicon 60 list, journalist & Silicon 60 researcher Peter Clarke hosts a conversation on startups in the electronics industry. Panelists Dan Armbrust (investment firm Silicon Catalyst), Andrew Kau (venture capital firm Walden International), and Stan Boland (successful serial entrepreneur, former CEO of Neul, Icera) join in the live debate.