when I place watch in gps test mode, I get -- -- how can i determine weather it is a concern with antenna or motherboard?

When/how did the GPS stop working? What firmware and GPS firmware is on it? Did you drop the watch? Have you opened up the watch already and check for moisture, dirt?

I received it used as a gift. I conducted factory reset. I am not sure it was working before I got it or not. I updated software to most recent possible. I haven't opened it up at all. I don't think watch has been dropped. It has been sitting for a few months and battery was dead when I got it.

What do you mean with "gps test mode"? What happens if you go outside with a clear view to the sky and select an activity which includes GPS like running or hiking? 

Don't open the watch and don't delete files from it, even when some suggest such things in the forum!

Because you wrote "GPS test mode showed -- --" I assumed you had put the watch it in diagnostic mode. Was that true? 

"Found an antenna that's known good" made me think you were preparing for replacing the antenna. Were you? Did you have a suspicion that the antenna is damaged?

Contact Garmin support first before such a last-ditch effort.

Do other components work? Vibrate motor, backlight, display, keys, temperature and baro sensors?

If the battery ran out and the watch was left for months, first see if you can pair it with a phone so it gets a new EPO file (satellite positions), then start an outdoor activity and leave it outdoors for an hour or even more. That way it won't time out but keep trying to acquire satellites. It can really take very long.