I've now done further fruitless searches on the web.
However, now that I have finally caught up with the recent updates to my GOG games, I decided to download the smallest of these weird named no file extension files.
It passed the listed file size check, so that's good, but of course failed the checksum test because there wasn't any.
I then tried to test with 7-Zip, because I knew it worked with PKG files, which these GameData_1_* files appear to be related to. That failed, until I used the open with # option.
Here's the results, which appear to be okay. But I am a bit dubious about all those SWF files, having seen similar before when 7-Zip cannot fully read something. Clearly I need to download the other two files and see what they report.
7-Zip 18.06 (x86) : Copyright (c) 1999-2018 Igor Pavlov : 2018-12-30
Scanning the drive for archives:
1 file, 1925265982 bytes (1837 MiB)
Testing archive: G:\GOG\GoG\The Witcher 2 - Assassins of Kings Enhanced Edition\GameData_1_ac
--
Path = G:\GOG\GoG\The Witcher 2 - Assassins of Kings Enhanced Edition\GameData_1_ac
Type = #
Everything is Ok
Files: 49
Size: 1925265982
Compressed: 1925265982
7-Zip 18.06 (x86) : Copyright (c) 1999-2018 Igor Pavlov : 2018-12-30
Scanning the drive for archives:
1 file, 1925265982 bytes (1837 MiB)
Listing archive: G:\GOG\GoG\The Witcher 2 - Assassins of Kings Enhanced Edition\GameData_1_ac
--
Path = G:\GOG\GoG\The Witcher 2 - Assassins of Kings Enhanced Edition\GameData_1_ac
Type = #
Date Time Attr Size Compressed Name
------------------- ----- ------------ ------------ ------------------------
..... 1712355184 1712355184 1
..... 42647 42647 2.swf
..... 65206 65206 3.swf
..... 31486 31486 4.swf
..... 13510 13510 5.swf
..... 34370 34370 6.swf
..... 34106 34106 7.swf
..... 38612 38612 8.swf
..... 54013 54013 9.swf
..... 12919 12919 10.swf
..... 26565 26565 11.swf
..... 32338 32338 12.swf
..... 25279 25279 13.swf
..... 264905 264905 14.swf
..... 33131 33131 15.swf
..... 38305 38305 16.swf
..... 57031 57031 17.swf
..... 114977 114977 18.swf
..... 57641 57641 19.swf
..... 122981 122981 20.swf
..... 92730 92730 21.swf
..... 5484954 5484954 22.swf
..... 21422341 21422341 23.swf
..... 453240 453240 24.swf
..... 112252 112252 25.swf
..... 106214 106214 26.swf
..... 452536 452536 27.swf
..... 174041 174041 28.swf
..... 16193493 16193493 29.swf
..... 249917 249917 30.swf
..... 3194758 3194758 31.swf
..... 380723 380723 32.swf
..... 1496815 1496815 33.swf
..... 309650 309650 34.swf
..... 176596 176596 35.swf
..... 3520770 3520770 36.swf
..... 66544 66544 37.swf
..... 25448 25448 38.swf
..... 3746599 3746599 39.swf
..... 1603 1603 40.swf
..... 246227 246227 41.swf
..... 422 422 42.swf
..... 59557 59557 43.swf
..... 1272656 1272656 44.swf
..... 1447929 1447929 45.swf
..... 9406 9406 46.swf
..... 107712 107712 47.swf
..... 346833 346833 48.swf
..... 150658810 150658810 49.swf
------------------- ----- ------------ ------------ ------------------------
1925265982 1925265982 49 files
I guess that having passed the file size test and now seemingly the 7-Zip one, I probably don't need exact details, to decide the file is probably okay.
One day perhaps, I will find out fully, if I test on a Mac ... my daughter currently the only one with one in our household, and I doubt she wants me to play around with it ... perhaps if it was a game more to her taste.