Over six months ago, I backed up the Cyberpunk 2077 folder as it's 59 GB and that would take a long time to re-download on a 43 Mbps connection.

How can I restore the game?

I have tried starting the download again via GOG Galaxy, then closing the program and overwriting the new Cyberpunk 2077 folder with the backed up one I have. However, when I relaunch the program, it wipes the folder and carries on downloading from 0%.

This method works in many other clients, such as Steam.

Can anyone assist?

You need to "import":
https://www.gog.com/forum/cyberpunk_2077/can_someone_explain_to_me_the_giant_patchsizes_compared_to_steam/post4

cancel the download,
set the game build to match whatever copy of the game you have copied over [may need to abort downloads again... depends on auto settings but just kill it if it starts]
then verity \ repair files for the game... Fuckpunk takes about 5mins to scan and should download 200 mbs or so if you don't have profiles installed

may want to restart your system but not needed if Windows 10 or 11

p.s, contact support... thats a funny one... you should hear back from them by 2033 ;)