Hey, i am downloading The Witcher 3 right now, but i dont think i will finish the download today. My question: Is the Dowload-Progression getting saved? So when i start my computer again tomorrow i can continue the Download from where i stopped it?

Depends on how you're downloading. Galaxy and GOG Downloader should be keeping your progress, even if it doesn't seem so when running the program. Browser downloads will not, unless you use an addon that gives such functionality.

I think Firefox automatically saves progress now.

Issue used to be that because of the session id the ADN got, the browsers were seeing the files as different, thus not resuming them. You could always pause and resume the download in the next few minutes (and I do mean few), since the session id was still valid, but pausing the download and resuming it next day would initiate a new one. If that behavior has changed without the need to use Down Them All, then that's nice to know. If not, that falls under the addons I mentioned above.