So hero units are mentioned in the trailer, but not in the summary or feature list. I know that this is a premium feature that was added to the Flash version some time later in the development, so depending on development shenanigans, they may or may not be included in this version AFAIK.

So are the in the GOG release or not? And/or are there any other big features present in other versions that GOG users won't get?

I went ahead and bought it based on your comment and another like it in another thread. Heroes are all there, and are unlocked at certain stages of the campaign. I figured there would be a lot of very low reviews and general rage if the GOG version were somehow incomplete, but it wasn't totally unfathomable that the DRM-free version would be intentionally crippled, and I wanted to be relatively safe.

GOG, you should update the game page. The current description conforms to what Kingdom Rush was a few years ago, before this content came into the picture!

Does this version have Hacksaw, Oni and Thor? Or is that mobile only?

All 12 heroes from the iOS version are here. It doesn't look like the wiki has been updated to even mention a PC download version, as far as I can tell.