You should be able to revert to the previous version; I did so mostly because the performance seemed worse, and after reverting it was back to where it was before the update.
I use the older version of the GOG launcher (1.x), but under MORE, I selected settings and switched back to 1.3.57 (though I have the offline version as well).
My GOG Galaxy gives me the option to go back to the Jan version, but there were 2 updates/patches released post Jan and prior to this last debacle.
I did start with GOG version 2.0 a few months back but I got tired of the automatic updates despite having explicitly disabled that feature. I have a good deal of the offline installers, but I would find Galaxy 2.x would try to update the game while I'm part-way through installing the patches, so I needed to shut down Galaxy while I installed my games. After getting sick of it, I just rolled back to version 1.x
It's bizzare that the older GOG launcher allows me to roll back a single patch, but the newest GOG launcher doesn't. If you don't care about achievements (though they still seem to partially work at least), you could try rolling back to GOG version 1.x; though it's a bit of a pain to install as it isn't really supported anymore.