GOG can and has made pre-loading available, specifically for the release of
The Witcher 3. They've done this (IIRC) by making an installer consisting of only a small executable, then offering the .bin files for download and only adding the executable on release day.
Witcher 3 was AFAIK the only title that has gotten a pre-load on GOG (probably due to a combination of its size, its popularity (= # of downloads on release day) and it being developed and published by another CD Projekt company), so they probably don't have a process in place to do this easily and don't offer it to other developers/publishers... yet.
Now, if you want to change that, this subforum is probably not the best way to go about it - instead, how about adding it on the
Community Wishlist for Site Features and then making a post in the general discussion forum about it :-)
(and yes, us backers who chose a gog key aren't getting pre-load either. And honestly, pre-load is nice, but I take not having to deal with steam over it on any day of the week)