Trine 2 Goblin Menace

No.

if you purchased goblin, then yes. Also, you can check via dlc tab of the game.

Trine 2 Complete story includes Goblin Menace, as the steam store says.

Trine Complete includes Trine 2 Complete Story, and Trine. That's both games + the DLC.

Go right click on trine in Library, and then DLC. You will see what you have.
If you have Trine 2, and than activated Goblin Menace, it should be now Complete story.

it seems you cant buy goblin menace anymore