You simply need to enter the following line of your desired file into your web-browser URL bar.
GOG.com Downloader still works like normal, if you input the right file name.

gogdownloader://game_title_game_or_dlc_file_name/installer_win_en

"game_title_game_or_dlc_file_name"
=
"Game installers and updates: / GAME BACKUP COPY"
&
"DLC installers: / DLC BACKUP COPY" items that you own on your account.

GAME/DLC BACKUP COPY e.g.:
gogdownloader://shadow_warrior_classic_complete/installer_win_en
gogdownloader://shadow_warrior_classic_redux/installer_win_en
gogdownloader://shadow_warrior_2013_dx_9_version/installer_win_en
gogdownloader://shadow_warrior_2013_directx_11_version/installer_win_en
gogdownloader://shadow_warrior_2/installer_win_en

Games and their DLCs are no different file wise, the problem is the "extra goodies".

I'm fairly certain that all games (that have "extra goodies") share a universal URL in a
similar fashion to the Games & DLC. Which downloads all of the "extra goodies" as a
batch, into a single directory.

Unfortunately I do not know the "batch" URL for them. I've tried the following examples:
gogdownloader://postal_2_extras/installer_win_en
gogdownloader://postal_2_goodies/installer_win_en (obviously these two would not work, my first attempts)
gogdownloader://postal_2_extras
gogdownloader://postal_2_goodies
gogdownloader://postal_2_dlc_extras
gogdownloader://postal_2/extras
gogdownloader://postal_2/goodies
gogdownloader://postal_2/dlc_extras

BUT you still can download them as single files the same way as Games & DLC.
But it's a grater hassle than the Games & DLC.

And unlike the former, goodies have titles that can differ from their game title,
depending on which version of the game it comes from.
And they don't have a unified file designation,
just a random number sting (usually five numbers).

gogdownloader://game_goodie_file_name/#####

These are some "simple" examples:

gogdownloader://the_witcher/10473
gogdownloader://the_witcher_2/10843
gogdownloader://the_witcher_3_wild_hunt_game_of_the_year_edition_game/70013

gogdownloader://aragami/71333
gogdownloader://baldurs_gate_enhanced_edition/36313
gogdownloader://baldurs_gate_2_enhanced_edition/36343
gogdownloader://x4_foundations_game/81721
gogdownloader://neverwinter_nights_enhanced_edition/81442

And some "pain in the arse" examples:

gogdownloader://aragami_collectors_edition_upgrade/71363
gogdownloader://aragami_nightfall/80372

gogdownloader://baldurs_gate_enhanced_edition_official_soundtrack/76813
gogdownloader://baldurs_gate_ii_enhanced_edition_official_soundtrack/76833

gogdownloader://x4_foundations_collectors_edition_content_dlc/81736

All other goodies for Neverwinter Nights: Enhanced Edition (and Stellaris).

This makes managing links for personal archival very annoying, tedious & inconvenient to copy/past.
Maby somebody can make a scrip that directs GOG.com Downloader
to a user made txt file with all their desired links? Or something to the same effect.

Another unfortunate consequence is that Alerts from GOG.com Downloader will no longer trigger,
letting you know that a Game in your Library has had it's offline installer itself been updated. :(

Soooooo, this kind of sucks, but it's better than nothing!

Oh, you can also still download your MOVIES through GOG.com Downloader as well. ;)
gogdownloader://video_game_show_the_witcher_3_wild_hunt_concert/en1video1

So for those of us that use GOG.com Downloader, WE STILL CAN!