I was trying to get the peaceful ending while romancing Clarmont with the Arland princess background, and after multiple resets I managed to do it! You don't need to befriend Gisette, but it seems like having a semblance of respect from her helps. I managed to get full success during the play and played opposite of her as Serah, which greatly boosts all approval and earns a fair bit of respect. I had to consult the guide for the stat checks, so this is probably going to be a lot easier in story mode. Hope that helps!