I was just installing the latest Into the Breach patch (latest offline installer released yesterday), and the installer splashed an advert for Duke Nukem 3D on the screen.

Could this mean that Duke Nukem is finally returning to the storefront? One can hope, I guess.

Aren't these installers offline with no internet connectivity? So this advertisement could be from that time, where Duke3D was still sold on GoG? I think those ads are outdated or are there new games shown as well?

Yes, it was the offline installer, but it's a brand new installer (just released this week) so all the adverts should be up to date.

GOG doesn't seem to remove ads from those installers once they're initially built, even for games that they no longer sell. There were also installers that had ads for one or more ARMA games, and those were gone for years; I'd be willing to bet there are other delisted games that are still being advertised the same way. I wouldn't read much into it.

Does nobody understand what I mean when I say the installer is brand new? I.E., this installer does not pre-date the removal of Duke Nukem?

This *IS NOT* an old installer.

It doesn't matter when a particular game build was packaged -- when they update an installer for an existing game, they only update the files for the game installation itself (including any files that Galaxy needs to interface with the game), and probably any license documents that they have to ship with the game. Everything else is just part of some template that they use to build the installers, and the ads are part of that template, as GOG don't seem to change them (that I've noticed) after the game's initial installer build (though which games are shown in an installer's ad carousel does vary from game to game -- seemingly based on genre -- and some show way more different games in their installer than others).
Hell, GOG hasn't even bothered to update the version of the free software they use to build the offline installers (InnoSetup, I believe) in many years. And someone pointed out a couple years ago that clicking the "download now" button on any of those game ads attempted to open a GOG site page in Internet Explorer specifically, even if it wasn't set as the default browser on your system. I haven't checked lately, but I don't know that they've changed how that works, either. (Not even sure what would happen on a system without IE, like all Windows 10 & 11 systems should be by default.)

Hmm, okay.

How about the fact that Duke Nukem was removed in 2015, while Into the Breach released in 2018? I find it hard to believe that GOG would keep and advert in rotation for a game that was removed from their store 3 years prior.

I've been hanging around here for eight years now, and very little surprises me when it comes to their cutting corners on things they probably deem not important enough to waste time fixing. :)

New '22 releases comes to GOG also have Duke3D ads in installers. There is always a chance. If ZOOM able to deal with right holders, GOG can too if they want.