Wondering the point of buying this game from GOG when all the mods are Steam Workshop!?
If it doesn't bother you to buy from Steam, do it. I don't like Steam myself, and no amount of mods would convince me otherwise.
In any case, you can download mods using various workshop downloaders. I like GreaseMonkey (for Firefox) and Enhanced Steam Workshop Downloader 2.1 from greasyfork.org (note that this doesn't work with every game, but it does work with Invisible, Inc). That combo puts a "Download" button under the preview images. I have verified this works by downloading a couple of random mods and unzipping them into a new subdirectories (any name will do) of the "dlc" directory (there may be a better place to put it, but I don't want to figure it out).
On the other hand, if you want to develop mods, you're probably better off going to the Klei forum.