You'll get 7x (random number since i'm actually very bad with numbers) content as the Steam edition.

So here is some of what you'll get:


• Restoration of Erathia, Armagedons blade, and Shadow of Death, Armagedon and Shadow of Death campaigns.
• Greater map pool.
• Random Generated/Randomized maps.
• More campaign options.
• More items.
• Conflux Town
• More creatures.
• HD mod
• Lot, lot, and lot of mods.
• Other stuff.

Now, what you need to do is simple, since i own the GoG version, i'll do it with the GoG version, the files from the place you got it may be different but it should be similar.

Take everything that's in the folder of the game that has everything, for example, mine is located here: \GalaxyClient\Games\HoMM 3 Complete , if you got the GoG version installed using the Galaxy Client, it should be at the same place.
You'll place all of the files inside that folder where your Heroes of Might & Magic III HD/Steam edition is at.
Should look like this: SteamLibrary\steamapps\common\Heroes of Might & Magic III - HD Edition.
Once that is done, rename HOMM3 2.0.exe to HOMM3 2.0.exe.bak or whatever you want, you can even remove it if you want but i'd keep a copy in case you want to return to the HD edition which i honestly wouldn't so i have personally removed it but it's up to you.
Copy the name "HOMM3 2.0", make a copy of Heroes3.exe which is the executable name for the GoG edition.
Rename that copy to HOMM3 2.0 and that's it.
Do not touch the launcher or else it won't work.


Important notice: The only problem i encounter is that the Steam overlay doesn't work, perhaps i'll be looking into it at some date later, the HD mod and the factions mods seems works flawlessly for me.

For the mods, look here: https://wiki.vcmi.eu/Mod_list and here: http://heroes3wog.net

And i'll be looking for a fix later.
If you find out how to make it work before i even try to look, then feel free to share your fix/workaround with us.