The website. WTF.
OK. I only spot checked some yesterday. Looked at more more today and some do show version numbers, some don't.

System Shock 2 and Trine show them and have a changelog. Saints Row 2 only shows (GOG-5), which is meaningless to me, but I assume it's the 5th revision? Outcast 1.1 seems to confirm this, as it shows version numbers for patches, one older as GOG-22, the newer one as GOG-23. Planescape Torment, no, but does have a changelog. Septerra Core, yes. Sanitarium, no.
Still not seeing any logic in what does and doesn't get a version number. Fuck it.