Sniskerbobberton: Perhaps GOG are testing said patch/update? Although, Team17 are renowned for shafting GOG users, I wouldn't be surprised if it takes weeks to get here.
That shouldn't impact the patch from being released to Galaxy users. Devs can now release directly to users without any testing by GOG. Only the standalone installers are impacted due to testing and the patch is currently not on Galaxy.
If this patch is late, then it's most likley squarly on the devs as GOG has completly improved their patching process.
I imagine it may be due to the GOG build having some issues not in the Steam build. Some of the fixes may also require changes to Galaxy features/API so that could be why they need a few more days.
Still not a good excuse though as patches should be released at the same time, but I can understand not wanting to hold up releasing on Steam if the GOG build needs a little extra work.