If you want to be extra offline you can skip using GOG Galaxy alltogether - just login to GOG through the browser, find the game in your library and you can download off-line installer. Still GOG Galaxy is probably easier to use for updates.
Luckily for you, major updates to BG3 don't happen frequently - every 2 months at best - that said once they drop you will most likely have to redownload most of the game due to scope of changes.
