As far as I could read over at Beamdog's forums, GOGs version of Siege of Dragonspear does not support mods. Can someone who already bought the game check this, please?

This is true:
Both Steam and Good Old Games deliver the DLC content in a zip file that contains all of the new content. This includes the new dialog.tlk file that contains all the text for both the game and the expansion. Weidu, the modding tool that all mods use to inject their content into the game, cannot access the files inside that .zip and the maintainers claim it's not practical to add that capability to the tool.

What this means, is that you will never be able to add a mod to BG1 once you've installed SOD. There is some talk about creating a mod that will move the SOD content from the zip file into the normal game files.

The version that Beamdog sells on their website does work with mods because they don't use the zip file method.

You can find out more on the Beamdog forums and at the Spellhold Studios forums.

there is more to it than that but yes... mods for vanilla or ee won't work with Dragonspear and you need an un-modded bgee to install it on

Thank you for the information. I hope GOG finds a solution to enable mod support in their distribution scheme. Mod support is essential for me to turn off some annoying things.

I did open a wish:

https://www.gog.com/wishlist/games/mod_support_baldurs_gate_siege_of_dragonspear

From what I understand he's explained it perfectly. Steam and GOG versions don't update the bg:ee dialog.tlk, instead it stores the new one as a zipped archive elsewhere. Mods will install happily (editing the original .tlk) but the game will draw from the zipped version instead, leading to new or edited items having lines of dialogue for their descriptions and mod injected conversations breaking, amongst other things.

I debated it for a while but ultimately decided on buying the beamdog version as it doesn't sound like a solution will be coming any time soon. It's only £20 for both bg:ee and SoD. Alternatively, if buying something twice incites you to rage, don't install SoD straight off. Mods will work fine with bg:ee and the 2.0 update, you just lose out on the new black UI. It won't solve the issue of mods not working with SoD, but maybe the time it'll take you to complete bg:ee will allow modders to create a fix.

Ta-Dah!
https://forums.beamdog.com/discussion/50441/modmerge-merge-your-steam-gog-zip-based-dlc-into-something-weidu-nearinfinity-dltcep-can-use

I can vouch for the Modmerge program personally. You just download it, run it, and you'll be able to install any mod that's compatible with the Beamdog version of Baldur's Gate Enhanced Edition. So far there seems to be only one exception, but it appears that the issue is with the mod itself and not Modmerge. Check out my posts in that thread to know what I'm talking about.

That's a huge step in the right direction, but I've read in that thread that the directory structure of the gog and steam version is still different from the beamdog version after using the tool. Unless beamdog fixes that problem, we will end up with more modding problems.

Honestly, Beamdog should have just made one version of the game and distributed it to Steam and GOG. It doesn't make sense for Beamdog to compile all the data slightly differently for the versions shipped outside their specific client. I'm betting sometime near the end of the month, there will be another patch for BGEE that hopefully fixes mods and a few other things brought on by patch 2.0 and Siege of Dragonspear.

I am glad to see there is already a workaround. Nonetheless the problem should be fixed by its root. Having three different version of the same game without a benefit, but additional trouble is just stupid.

What is the reason for the zip file anyway? Is this how DLC is required to work on Steam and GOG? From a technical perspective, I would think the easiest thing is just to make sure that people with certain DLC have other versions of certain files + additional files. So I don't see why they would now be forced to put everything into zip files. I guess it compresses nicely this way.

The installer is compressed anyway. This can not be the reason to drop in the DLC as zip.

The zip file might even decrease the compression ratio (note: the argument isn't valid if the zip file is uncompressed): the zip compression algorithm is rather bad in modern terms, but it can cause problems for the second stronger compression algorithm that may result in a larger overall file size. It is easier to compress uncompressed data and each compression step causes overhead.

There are some combinations of input data and compression algorithms where it can be useful, i.e. compressing the output of a good predictor, but generally, it's not a good idea.