I've set up a gitlab repository for the firmware, ppssync, and cabled the prototype. Below is an artistically blurred picture of it:

The external antenna is for testing purpose only, the GPS is quite capable to fix indoor with its ceramic antenna. Here's the hookup guide between the Trinket M0 and the GPS board:

The trinket provides the 3.3 V power (from onboard regulator) to the GPS, greatly simplifying the set up! Thanks, Adafruit!