I also went ahead and installed (and uninstalled) both the new and earlier installer versions one after another, and with a quick check the differences I could see were (the earlier version is a couple of weeks old download, I think it was updated on 25th of January):
- Yep, that Paradise Lost menu option has been added. If you click it, it tells you to check your installation (as obviously the DLC is not (yet?) on GOG). That option was not there in the older version.
- The Performance Options seem to have changed somewhat. I checked them quickly because the 5021 changelog (link on the Steam Postal 2 pages) mentions that some non-functional Level of Detail-options have been dropped from the Performance Options in the version 5021.
- The old version runs in fullscreen by default, while the new version runs in a window. Is this better for e.g. Windows 10, or multi-monitor setups?
- The new version ran some Visual C++ 2005 checks and installations at the end of the installation (the old one didn't seem to), which might explain the increased size of the installer. Not sure if there were other dependency checks/installations as well.
All in all, to me this indicates the GOG version just got updated to a "Paradise Lost"-compliant version of the game, which might mean the DLC is finally coming to GOG. At least the publisher was finally able to get some upgrade to the GOG version...
Anyway, even if the GOG version is 5021 now, it seems to be one step behind the Steam version which is apparently currently 5022:
http://store.steampowered.com/news/externalpost/steam_community_announcements/2079952210072387432 EDIT: I am only assuming the GOG version is the older 5021 because that is in the installer file name... but I'll double-check it again, if there e.g. is a subtitle option in the Audio Options (in which case it would be 5022, and the 5021 in the filename is a typo).