I doubt that there are any retail games that come with a gog registration key aside from games made by CD Project Red. Therefore, it is most likely a steam only key. But that's just my idea on it.

It is a Steam-only key (at least for now, unless it somehow ends up in something like GOG's Reclaim Your Game program). The collectors edition predates the GOG release of Terraria. But, I bought it anyway even though I have the GOG version since I like having a boxed copy of my favorite games, and for the goodies. One other thing to note is that the CD version of the game is quite out of date so it would need to be updated anyhow.

Must've been from when the devs thought they were done updating the game...before Redigit changed his mind. :)

When the CE was published, Terraria was only on Steam. You could get it on GOG later through GOG Connect, provided Re-Logic chooses to participate. (I wouldn't be concerned; I bought the game on both Steam and GOG anyway. All the content updates made it worth it.)

There is no real difference, but the rabbit pet (which one u can activate easily with use of regedit to make ur GoG ediition Collectors)