I can see the option to download/install BG3 via. GOG GALAXY.

With other GOG games, I have been able to download the full game for offline installation. Is this possible with BG3?

IMHO. If I can only install via. A supplied online installer, I am unsure how GOG can compete with Steam.

Thanks

You can.

However, be warned that there is usually a delay in updating the offline installers and the offline patches. I cannot tell you how long the delay will be because it varies a lot. On BG3 release version they have been updating on up to 1 day delay. That's actually pretty good for GOG in these days.

Also, nowadays, they only ever have the one latest (that they have currently posted) full installer up at a time. When they post a new one, they take the previous one down. Likewise, they only keep up a few of the more recent patch files.

There are some more warnings about the disadvantages in relying on the offline files as well, but they become a bit complicated. (the naming of the patches for BG3 is a confusing nightmare, for instance)

-edit- Also, forgot to say: this forum is actually more geared for the prior BG releases. You can get a lot more BG3 interest in your posts (and help) if you go to the BG3 forum.