Wikipedia called 2016 Shin Godzilla a "reboot".
Does that mean I won't lose anything by not watching any previous Japanese Godzilla movies? (in terms of universe/continuity knowledge beneficial to watching the new film)?
Wikia also calls this a reboot
Shin Godzilla was co-directed by Hideaki Anno and Shinji Higuchi, with a screenplay written by Anno. The film acts as a reboot to the series, and details Godzilla's origins, and his arrival in Japan for the first time, while also showcasing the human response to the disaster.
Since the plot goes on to detail Godzilla's origins, it's clearly cutting continuity with the previous films. It's worth noting that canon in the previous films has been spotty at best (i.e. Hedorah no longer exists), so it's simpler to make a clean reboot so future films are not burdened.
It's a clean reboot. You don't need to know anything about previous Godzilla films, any of the characters, any of the events, or anything else of the mythology. You might get a bit more out of it on the meta level with references and the odd in-joke, but nothing in the actual film requires prior knowledge.