Some days ago, the FLAC sountrack received an upgrade on Steam. You can read about that here:

steamcommunity.com/games/692850/announcements/detail/2889580151788855030

Apparently, they fixed some audio issues from the previous version. The upgraded soundtrack is 870 MB, the previous one is 830 MB.

Seeing that the Gog sountrack is still 830 MB, I guess that Gog has not updated the sountrack yet. Will it get updated?

The GOG version still seems to have the shitty broken 100% Completion that was introduced in the Steam version initially but wasn't fixed in the GOG version as far as I'm aware.

The GOG version has already been fixed for quite some time now. I'm not sure why the file size is different, though.

Here's the proof that the GOG version is already fixed:

https://www.reddit.com/r/Bloodstained/comments/cxlpcz/the_flac_version_of_the_ost_is_now_actually/

Yeah, I know about that, the thing is that after fixing that they updated the soundtrack a second time on Steam, supposedly upgrading the audio quality. Check the first post's link.

So yes, right now GoG's FLAC version is already truly lossless, but on Steam they updated the sountrack a second time (that's why there is a filesize difference), and it is supposed to have better audio quaility.

And seeing that that was months ago, it seems that in won't ever be updated on GoG.

No, the GOG and Steam versions have the exact same audio quality currently. Steam got a second update to it's FLAC version because the first update to the Steam version wasn't actually lossless, and Steam users were stuck with lossy versions up until now.