Thanks for all your answer.
@Tim Cook. No i didn't freeze the track

@Guitarhacker. I try to regenerate the piano part many times. It seems the part in G and the one in Bb are just totally different
As for you advice to record the song having the right key. Generally I can't because i don't know who will sing it in the end
The second version of Come & Go as my ^préférence, really good song by the way

@MarioD. I didn't try to transpose the part in G, 3 half step up will add artifact without doubt. I never use the midi transcript, 'i will see if there's one