Does this ESO Morrowind Upgrade Work on Steam?
The versions sold by third party retailers listed above (including GMG, DLGamer, BundleStars, and GamersGate) are all Zenimax/Elder Scrolls Online Account version. Namely, you'll activate the game here.
Having said that, if you have purchased a Steam version of ESO, it WILL still work with the deals listed above - as long as you LINK your Steam account to your ESO account (which you can also do on the login page here). More details of linking/syncing your ESO accounts can be found here.
Once you've done that, you'll be able to use the product activation code provided by GMG et al. and simply activate the game in your Elder Scrolls Online account dashboard, or through the Zenimax Launcher.
Pre-Ordering Morrowind Upgrade: Early Access and Bonuses
If you bought the Morrowind Upgrade deals listed above, or the Collector's Edition Upgrade - you'll be eligible to receive early access which started on May 22nd. Yes, this includes deals from third-party authorized sites listed above.
All pre-order purchase also includes the Morrowind "Discovery Pack" which contains some nice additional trinkets and cosmetic goodies. You'll receive the warden costume, treasure maps, experience scrolls, and dwarven crown crate. The best pre-order bonus though is the dwarven war dog pet, which is available only as a Morrowind pre-order bonus as of writing.
This is the base ESO game + Morrowind Expansion.