is the GOG store version HD as well?

It won't let me paste a link, but go to the store and look for "Heroes of Might and Magic® 3: Complete".

No. It's the original Complete version.

You can download and use the HD mod
https://sites.google.com/site/heroes3hd/

Note that the "HD" version sold on Steam is actually a real crappy port with tons of issues. GOG's version is the original DVD complete version, and if you prefer HD, the mod above is far superior to Steam's "HD" version.

Oh awesome, thank you :)

would also like to add that the HD version on steam contains only the original campaign "Restoration of Erathia" , it doesn't contain the 2 expansions so it's a really bad deal in my opinion