MJim: I tried just now to download it again through GOG galaxy and I had the same issue.
It does look like I was able to download the full 2.1GB using MS Edge through their website directly; it looks like the name is a bit different as well.
bg3ost_wav.zip = Bad file only ~600MB download through GOG client (13 Files)
bg3_soundtrack_wav.zip = Good File ~2.1GB download through web browser (43 Files)
SleezureTime: Yes it looks to have been corrected. I was getting the same result from the web and Galazy earlier--the broken 600 MB archive. But it seems the OST WAV was reuploaded now. Works on Galaxy.
Yeah, it's working as well from GOG Galaxy; seems the name has changed to the one on the website which I was able to download earlier, so it is now
bg3_soundtrack_wav.zip instead of the problematic one
bg3ost_wav.zip In any case thanks to those at GOG that fixed this problem, much appreciated!