Hi CandyT! Thank you for playing the game, I'm so happy to hear that you liked it! >\\\<
And no, you're not dumb! The endings meet requirements to show up. To get the option to leave Gareth to his destiny you must find the portrait of his lover before you enter the servants' room where he's hiding. To stab him instead you have to retrieve the knife from the corpse in the hidden room, it will do it automatically no matter the choice if you interact with it before leaving the room... let me know if it works :)