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 re-download most of the game due to scope of changes.
Yeah I understand the Like or want the update's for sure. But for me I mean a 10 gig patch will be like 10 to 11 hours or more to download. (Thank you for taking time to teach me something on the Gog web purchasing.)
I will probably put the updating off until I can carry my system over to my Friends Home and get it.
I sort of wish game patches were like they use to be, I remember you could download patches in the back ground, and install them when you got ready. Now a lot of places force Patches on you, and sometimes the patches cause more trouble than good.
I remember I use to wait and read the feed back on patches a lot of times before I committed to them.