Yes, the pretty picture says it has a season pass, but the GOG info ONLY shows the useless fluff actually in the package.
Anybody can confirm that all the other expansions become available?

I can confirm no problems here. I bought the season pass after the first (of 3) DLC's were available. The Revelations DLC was there to be downloaded and installed with the main game, together with the season pass fluff.

The second DLC, Invasions, has become available today (when scheduled) and downloaded and runs fine.

I don't yet know when the third DLC is due, or what is in it, but I would expect it to be worth it and on time.

If you aren't sure what the DLC's actually contain just have a look at their store pages as if buying separately. (This seems obvious, so perhaps I misunderstood you).

Hope that helps.