To be honest I'm only really a fan of one set rate such as 40% to be honest, most of the "Save Up To" ones that GOG do, I don't bother to buy from. GOG's the only online retailer I use that I can remember doing these save up to deals.
As Madcartoonist says, getting a bigger discount for buying in bulk is certainly nothing new - what is slightly unusual is GOG's ways of doing it. And for my money, they do it better than most other people.
Normally a promotion for a set of games is either this: You buy the entire set of games and you get a nice discount, if you buy just a few of the games you get no discount. Or, you might get some discount on the games if you buy just a few, but if you buy the whole pack you have to buy the games you already have again - otherwise you won't get the big discount.
Look for instance at Steam. I don't have the big problem with Steam that some people have here, but I have a lot more trouble with how they handle some of their promotions. If there is a promotion for a certain publisher for instance, it's usually something like this "Save 10-50% on select games from this publisher, or 75% on the complete publisher catalog pack". This is similar to what GOG usually does, but with one big difference.
If I have every game but one from that publisher, I get the full discount on GOG for buying just that one game. On Steam there is no such consideration. And if I buy the complete pack when I already own some of the games I only pay for the games that I don't already have on GOG, but on Steam I have to pay for all the games - but get no extra copies for the games I pay for again, unless it's a very special promotion.
So I don't agree that what GOG has been doing is unique, and I'd say that they do it better than many others. And as some have said I don't see anything wrong at all with getting a higher discount if you buy more - isn't it just natural for a business to reward good customers?