There are a number of games that directly support the creation of user generated mods, and a plethora of others that have modding tools in spite of the game maker not directly supporting mods.

I would like to see a category/feature listed in the Game Features section of a Games Description page which identifies games which have mod creation support, either directly or indirectly.

Thank you for your consideration.

Edit: updated text in italics.

All games (excluding online only) have the ability to be modded. Modding simply means to alter the distributed files somehow. This could something simple like editing text files in M&B to utilising the full engine creation tool which the developer used (for example elder scrolls, or things like the doom 3 engine reimplementation).
So to have a category for this is going to be a lot of effort on their part, and probably not accurate, for no gain. If you are interested in modding (what’s left of it from workshop and creation club) then look over on moddb or nexus or one of the game specific ones.

Your assumption laden condescending dismissal aside, I wasn't concise enough. I will use the imprecise vocabulary of gaming as my excuse. I say "modding", and you and pretty much everyone else immediately thinks "using mods". I am a bit older than most, and use the word "modding" in its original sense, creating mods.

Not every game supports creating mods directly, and I do not know every game which has tools for making mods. That information would be helpful and meaningful.

1) Create a community wishlist entry for your request, and link to it from your OP.

2) You dismiss nightcrawler.488's response in a way that implies you didn't even read it. To repeat, All games have the ability to be modded, at least indirectly. I, too, do not agree with this statement, but for a different reason: some games require a network server as part of their gameplay, and may use DRM to ensure you can't play after doing any appreciable modding, or have sufficient content server-side that you can't modify the game appreciably at all (e.g. Stadia). However, this is not, as far as I know, applicable to gog (unless you include Gwent, maybe), so adding a flag to indicate if a game can be indirecttly modified would simply flag every single game.

You should at least limit your request to "using official free/included tools, to the extent that those tools allow" or "which have already at least been modded once". The former might be useful, but the latter seems soft of useless to me, since many modders do not share their information, so you can't make mods of your own based on their work.

That label is so infinitely broad that it is effectively meaningless, and therefore, would not be worth listing on any store page.

You don’t seem to have read what I said. To create a mod oh are modifying the distributed files, this is where the term “modding” comes from. Be it replacing a texture or model, hacking an exe, creating a level, it’s possible n any game.
Take NWN which comes with the official toolkit. This also has unofficial exe hacks which extend the engine. Same with elder scrolls which has skse which is an unofficial script editor.
I do not mean using mods at all, coming from days of hacking code whilst it is loading on a spectrum I am well aware of what modding is thank you!
If you want a tag high states “which games coke pre bundeled with official modding tools” then fine, if as I say, that will be time consuming and not very accurate, it would highlight the disparity of the product between GOG and steam, which you can already see just by going on moddb and looking at some of the mods there which are nothing more than steam workshop adverts.

As others have told you, that's an impossibly broad label. I could take a hex editor to Mega Man 2, start typing wildly, and change the colors of the stage. That's technically a mod. I slap a picture of a butt into Starbound and define it via JSON and LUA? A mod as well.

Which games are modding viable is still too broad of a question. Does that account for source ports (OpenXcom, OpenRCT2, OpenLara, OpenMorrowind, etc) or no? What is the depth and level of programming expected of the user? Would they have to explode the executable or files thereof?

Does it count as modding if it's nothing more than a glorified scenario editor? Jazz Jackrabbit 2 and the Railroad Tycoon games have level editors (and powerful ones at that), but you can't change critical assets in them; you can't suddenly fly airplanes in Tycoon nor can you change Jazz out for Mega Man.

You would have to literally define what a mod is (given that it is literally a contraction of the word, "Modification") and work from there.

In games like Metroid, you can attach a script to the game's memory to introduce features that was never in it to begin with, such as a pause menu or a map. But of course, that was never meant to be modified.