In the upcoming tab there is a game named "Cornerstone". The gamecard says that it comes out 26th of April 2016, but it also allows you to buy it with a 20 % discount. It`s NOT flagged as "In developement", so this is kinda weird!

It is just a normal pre-order thing.
https://www.gog.com/forum/general/preorder_cornerstone_the_song_of_tyrim_5053b/

Aww I was hoping this was going to be about the band Styx. :( haha

There was the game "J.U.L.I.A. Among the Stars" in the sale lineup, offered with a 60% discount. The only date listed on its page,
https://www.gog.com/game/julia_among_the_stars_coming_soon

is
Released: September 12, 2014

Otherwise it's marked as "COMING SOON", no price is listed. What's going on...

GoG is either going totally bananas, or.....

You can find the real gamecard here:
https://www.gog.com/game/julia_among_the_stars