I'm not entirely sure. I purchased Pillars of Eternity from GOG when it was released and had a hard time downloading the files. For about half an hour after it unlocked, I was still coming across very low speeds (10~50kb/s), when my usual download speed from GOG is around 15~20mb/s, and was encountering multiples instances where the download would just cancel due to connection errors. I decided that was it and left it for about 2 hours before coming back to it, which went fine.
I'm guessing the release of TW3 will tax GOG much harder than PoE did, which I'm sure CDPR know already and likely have taken measures to ready their servers. Still, I am expecting to run into some issues but hopefully what we'll have to download will be relatively small in size.