The GOG release is unfortunately not the best. The one in the FX interactive website at least has the expansion of King Arthur. When it was released, FX interactive was selling an anthology version including FOUR expansions, like the spanish reconquista. If would be great if GOG could get the approval to release all expansions available.

Hopefully GOG will add them someday; at the least, another user posted links for the three campaigns FX Interactive isn't selling anymore, though the campaigns are only in Spanish:

https://www.gog.com/forum/tzar_the_burden_of_the_crown/download_expansion_campaigns_officially_released_only_in_spain_and_italy

I made an English translation of the Reconquista campaign, though I haven't tested it all, so if you try playing it, let me know if you find any issues:

https://www.mediafire.com/file/3trs1ojl9fcic31/The_Reconquista.wcm/file

I'm currently working on translating the short Genghis Khan campaign as well, and then I'll probably do the others as well.