The issue has been found and addressed - new games should appear in the library without such delays.
Apologies for the inconvenience.

Main website problems have been fixed and it should be back to normal now, so newly purchased/claimed games will show up in your GOG.COM library (with the possibility to download offline installers).
However, we're experiencing delays in games showing up in the library in the GOG GALAXY app. Our Team is currently working on a fix, but I'm not sure how long this may take.

Actually, our Galaxy Team managed to fix this issue, but it may still take up to 30min before all accounts are back to normal.