This usually happens when the file is corrupt, mostly due to downloading errors.
What makes me wonder is, that you said you're using the GOG Downloader. It should make sure, that the downloaded file is okay, so it's supposed to be. On the other hand, also your file system or hard disk could produce this failure afterwards. The easiest thin to try, even if it takes a while is to re-downloading the dmg file, preferably to another location or even to another hard drive. Then check if the issue is still there.

I have similar problem with Windows and I wonder if and where should I report it...

I already re-downloaded files because md5 hashes (cheched them here - http://www.gogwiki.com/wiki/The_Witcher_2_Enhanced_Edition ) were not correct.
Both times I used GOG Downloader, and now hashes are still wrong (all but .bin w/o number). I can't check whether first hashes are the same as those now, because I've deleted "corrupted" files.
.exe downloaded manually has the same hash as .exe from Downloader (still wrong, e84aac0223834c5769a0fda3aa223947)...

Is there a possibility that GOGwiki has wrong hashes posted? Or that files got corrupted on the server...? D:

Yes, it looks like they're outdated (that's why I refrain from referring to them most of the time). I get also a different hash for setup_the_witcher2_ee_3.4.0.25-11.bin (0b0e08f0ac106f5e4af70903dce63714) and the exe has the exact same hash as yours (e84aac0223834c5769a0fda3aa223947).

I got same result for -11.bin... Could we compare our other hashes?
(We're hijacking this topic, lol. XD)
My hashes, already formatted:
e84aac0223834c5769a0fda3aa223947 *setup_the_witcher2_ee_3.4.0.25.exe
1ffa67383248c9ec37ef6fb42f05f8f7 *setup_the_witcher2_ee_3.4.0.25.bin
180c0c2645e91f827e78a220a934183c *setup_the_witcher2_ee_3.4.0.25-1.bin
a782a5605a9bc18bf52587d1bca59601 *setup_the_witcher2_ee_3.4.0.25-2.bin
30377077791d9fbfa2ce950e97828c7e *setup_the_witcher2_ee_3.4.0.25-3.bin
d6f6d7131b07c612aca58e252ac49bc7 *setup_the_witcher2_ee_3.4.0.25-4.bin
fe9aecfcad0a39a47cdeab80950fdb2d *setup_the_witcher2_ee_3.4.0.25-5.bin
bcca8cf38e87bf9cdb22630853c994ff *setup_the_witcher2_ee_3.4.0.25-6.bin
08d327c75fdbe95f820daaad28ebab01 *setup_the_witcher2_ee_3.4.0.25-7.bin
bac411bdb7d52c4e5a97bbb3b75b574f *setup_the_witcher2_ee_3.4.0.25-8.bin
55cd0eaa3331c7b6502ca5b86d31ee1d *setup_the_witcher2_ee_3.4.0.25-9.bin
0445d535688070f6288bbf01c22ef7d9 *setup_the_witcher2_ee_3.4.0.25-10.bin
0b0e08f0ac106f5e4af70903dce63714 *setup_the_witcher2_ee_3.4.0.25-11.bin
(^setup_the_witcher2_ee_3.4.0.25.bin has the same hash as on the GOGwiki.)

can some1 speak english on for what i should do because im no computer pro if theirs a step i should do i need a extremely broken down simple way to fix this like go to your downloads>witcher 2>file>aoeusgwagW> and be really really straighforward on how to do this because i dont understand any of the gibberish like this thanks

Okay, I'll try to explain it as simple as I can:
User DeMignon and I are discussing problem similar to yours, but in Windows installer.
DeMignon reponded to you in second post in this topic. He suggests that your file is just corrupted (damaged; there's something wrong with it), and want you to download that file again... which, as I now noticed, is a bit stupid to do, because Mac installer is just one big file, oops...

I googled your problem.
Either your file is damages, or it's problem with third-party software on your Mac.
Check this link: https://discussions.apple.com/thread/3801444
And if that does not work - here are GOG solutions for common problems: http://www.gog.com/support/the_witcher_2/ibread_first_mac_os_xb_common_troubleshooting_stepsi

Nah, not stupid, just a fix to the most common failure in this case ;-) But you're right, it's annoying and maybe not even the solution here, as the GOG Downloader usually prevents faulty downloads.

thanks for using english :) u guys r life savers