I no longer had the GSC files on my HDD, but I tried again...
I set up a folder C:\...\Cartes du Ciel\cat\GSC, in the GSC folder I make a folder of the same name as the first folder I want to copy from ftp, say for example S8230 (CSP area), so now it looks like C:\...\Cartes du Ciel\cat\GSC\S8230.
Then I go to
ftp://cdsarc.u-strasbg.fr/0/catx/bincats/GSC_ACT/S8230/ and copy all files minus the first one (named '=done=') into C:\...\Cartes du Ciel\cat\GSC\S8230, which then will contain 48 files named from 9490.GSC up to and including 9537.GSC, total 2.32 Mb (in this case).
In CdC catalogue settings I then check HST GSC compact and direct it to Cartes du Ciel\cat\GSC, and Bob's my uncle.
Attempts to copy folder S8230 into ...\cat\GSC lock stock and barrel will fail, as copying the earlier mentioned '=done=' file will generate an error message.
You repeat this process for all remaining N/S folders in
ftp://cdsarc.u-strasbg.fr/0/catx/bincats/GSC_ACT/ and it should work fine.
Just now I tried it for both v2.76 and v3.0 and I had no problems... let me know how you go