Hey, so after realizing how some developers screw over GOG customers by never delivering their newest patches to GOG versions of their games, I've started to be more wary about buying new games on GOG for fear of ending up with an effectively orphaned game without essential patches.

What are the easiest ways to tell what patch number the GOG version of a game is one? And is it possible to tell so without having purchased it beforehand? I tried looking around the storage pages a bit but couldn't really find anything.

That's something that's a bit annoying: you can't see changelogs/versions/patches unless you have bought the game. There are however helpful Goggers who do post changelogs in the game specific forums, so you could try looking there if there are any posted.

The fact this even needs to be asked is embarrassing for GOG.

Agree.
And while implementing pls add patch notes too :P
Oh and steam version number would be welcomed too if the game is available there.

Yes, by using the GOG Database. Search the game you're interested in and then go to the Downloads tab where you'll see the current version. You can also go to the Changelog tab, which has the dates for the changes that were made to the offline installers and patches.

Example for Titan Quest.

It'd be rather easy to implement a version number marker using human readable marks at that, which I dunno, is like 20 pt away from the game title in your library?

For example, House Flipper (Version 1.54) instead of having to look down and see the visual hash of the version.

Is there a way to verify this in comparison to the Steam version?

https://www.gog.com/en/game/aircraft_carrier_survival

https://steamcommunity.com/app/1021100/discussions/

https://steamdb.info/app/1021100/patchnotes/

Found https://www.gogdb.org/product/1870324793 which at least shows a version number.

What you're asking is not possible in general. GOG version numbers have been known to be incorrect in the past, and Steam doesn't publish version numbers, maybe for the same reason. Steam also has its own internal versioning.

You can sometimes find out the answer from community forums, changelogs, patch notes, etc.

https://www.reddit.com/r/pcgaming/comments/hzbtpv/how_could_i_find_out_the_last_version_number_of_a/

The game was published on gog in March 2023. (The build date for the gog version is February 2023.)
The last free update was released on Steam in May 2023.

Gogdb.org says it's version 1.7.4, but in-game the version number is displayed as 1.7.3.

You can ask somebody who owns the Steam game to create a sfv file of his installation folder
with quicksfv, for instance, and compare it with your installation.

If you have access to a machine with a gog and a steam installation of the game, you can compare
both with WinMerge.