We don't KNOW that.
Gosho can easily kill one of them or have a bittersweet ending where they get into a relationship BUT it doesn't last and they move on. (I actually like that second idea... a LOT.) Goodness knows it would be a better way to end it than "He turns back, defeats the organization, and gets the girl and they live happily ever after! ". The story would feel like it was worth the while. I would really enjoy that ending. Their relationship isn't really progressing. Emotions they have been feeling for a while are becoming known. That doesn't mean that their "love" is growing. XD
I don't think they should be together. At all. They don't fit. It's just... it feels FORCED. It's not right. I just don't think they belong together.
And for the last time... I doubt... and I mean REALLY DOUBT he LOVES her. There's NO WAY that he's in love.
He places her below his cases.
He always runs off.
He said LIKE at Big Ben.
They are TEENAGERS and while love has no boundaries and it doesn't matter the age, it is highly UNLIKELY for him to be in love.