If your aircraft was equipped with the GDL-88 ADS-B transceiver (or any other UAT transceiver) and the other aircraft was equipped with at least ADS-B Out (Extended Squitter transponder), then that traffic would have been visible to you long before ATC knew it was there. If both aircraft were equipped with a GDL-88, then both aircraft would have seen each other, regardless of what kind of ATC coverage there was. The ATC coverage is important for aircraft that are not equipped with ADS-B Out or with only a Mode C or Mode S transponder. In the case of the near-hit described above, if both aircraft were ADS-B equipped, this would have never happened.