Recently reinstalled my CD version, since last time I got stuck possibly because of a glitch. Now I was thinking of patching to the latest version, but sierra's help site mentions versions 1.0a and 1.0b, with a fan patch fixing some stuff only applicable to 1.0b. But according to the README file in my installation, I have version 1.1.

So I'm wondering, what version does GOG use?

Strangely enough the README file in my installation shows the version as 1.0a but it has a later date than the 1.0b README file in the attached screenshot.

The retail floppy version shipped in 1993 was v1.0; I don't know if the CD version shipped a short while later had a different version number. However:
v1.0b was the patch for the floppy version.
v1.0a was the patch for the CD-ROM version.
Possibly explaining why v1.0a might have a later compile date than v1.0b.
The SierraOriginals re-release CD-ROM from 1996 that I own states v1.1. I would suppose this is the version on GOG.com, although I haven't checked yet.