Ok I managed to make my game work again, so I redid the playthrough with Gilbert..
The Box? I did not check what's inside the box though?
Ah I see the options "Answer seriously" and "Make a joke" that not is mentioned in the guide... I chose "Answer seriously".
After that I chose "Admit" when Mark and Anders visited.
On Day 7 when I sit next to him I eventually "Agree" with him and also chose "Under one condition" and "My pendant".
Lastly I chose "Sit across him".
And with all these I got the kiss scene at the end!