Is this the game that GOG.com paid the developers to make a new compatibility patch, which ended up being GOG-exclusive? I seem to recall that being a sore point with folks who bought it elsewhere, so this is a nice resolution.
I wasn't aware of GOG paying Arkane to patch the game (which is not to say it didn't happen), but the rest is certainly true.
Based on comments made by folks at Arkane, though, I don't believe they ever intended v1.19 to be exclusive to GOG. I think that a project they were working on around that time was unexpectedly cancelled (or put on hold at least), which must have caused free work on an Arx patch to become rather a low priority, until they got things back on track.
I'm glad they've finally got things sorted out, and the source code is a real unexpected bonus. Hopefully Arx will live on for a long time as a result of that.