They can do so with these games: https://www.gog.com/games?sort=bestselling&search=gog&page=1

gog bears have stolen all the free games to buy cocaine-crack

Explains the replacement squirrel and sheep...

No! Say it isn't so! I thought they were into steroids with all the weight-lifting they have to do!

On a more on topic note, as Konrad said, they will be giving anybody who fulfilled the freebie terms before Wednesday, 23rd March,10:20p.m. GMT, which is when they put up the run out notice, their freebie.
And it did say in the FAQ that they could run out.
Although GOG did screw up in that they underestimated the popularity of the freebie offer. At least, Some people will be getting a freebie.
Commiserations to those that were one or two games away from reaching the total. I know I was 2 games away. But, c'est la vie. *shrug*
But I bought some good games at (a sometimes deep) discount, shortened my wishlist, and increased my backlog. At any rate, I was satisfied with my haul.

As it is, I suspect that the freebies will be fulfilled out of pocket for GOG, i.e. that they'll take a full price game and simply write off the cost.

that replacement had some kind of symbolism form me. since the bear represented the stability of their principles and their principles started to change since 2014, gog decided to take a squirrel. cause they squirreled their way out of no region locks, one price, no drm, .... so i'm very afraid of the possible symbolism of a sheep. does that mean gog becomes nice and fluffy to the pubs and does anything they want?
yes, they did and mentioned that it would be highly unlikely that they run out.

Maybe the next one will be an alligator. Or a Golden Retriever or German Shepherd to represent loyalty.

Others have answered this pretty well, but it didn't seem right not to respond personally. Basically, my answer is no, I don't usually buy things expecting something free, and I have only been buying games in this sale that I actually want (ones on my wishlist).

The problem, though, is that I entered this sale under a certain condition (free game at 10 purchases) and to have that condition removed when I was halfway there doesn't seem fair. So in that regard, when the deal has already been established, yes, I do expect my free game in return for my purchases.

But now that the licensing issue has been explained to me, it makes more sense. It just would have been nice going into the sale knowing this could happen, then I would have known the deal could change. Now I know, so I'll proceed with caution next time.

Edit: I did read the FAQ when the sale started and I noticed the part that said the free games may run out. I suppose at the time I didn't really understand what this meant. But to be fair to GOG, I have to admit they did state the possibility.