The time signal should be very accurate if it’s done competently. 🙂
There’s a caveat here that if you rely on the GPS unit spitting out NMEA data, that comes in ASCII format over a slow serial connection. By the time it’s made it down the wire and through your serial buffers, you’ll probably only get ~ 100ms timing precision.
The GPS chip will also produce a PPS signal, which is pin which gives you pips once a second, on the second. Using the GPIO lines on a RPi or Arduino, you can sample that at high frequency to get a very good time standard.
My Polish isn’t very good, so I don’t understand much of the attached webpage, but it seems to mention PPS towards the bottom, which implies this particular box ought to have much better than millisecond precision.