Use of the Rockwell "Jupiter" GPS module simplifies the circuitry by using it's 10 KHz output instead of the more common 1 PPS. The
price to be paid is slightly reduced performance. There is no capability for "hold-over." When the GPS signal disappears, so does
your accuracy.
You are left to your own devices to determine the suitability of the GPS signal, but my circuit posted here might suffice:
http://ni6e.com/time/GPS_Valid_Fix.html
Another simplification is the use of a Microchip 12F629 to perform the "divide by 1000" that is usually done with three decade-counter chips.
One 8-pin DIP package replaces three 14-pin 74LS90 chips. Source and HEX code for programming that chip can be provided.