I guess it depends on how you enjoy games? For me, I rarely even use a gallery other than to see if I missed something along the way. Finding or achieving that scene in the sandbox mode is not only sexy, but highly satisfying and enjoyable, and I also love it when I can replay scenes between new content by going back to see those characters again and again to interact with them. For me, that's one of the biggest upsides of the sandbox format over a one-directional, narrative VN, which often makes me lose interest.The problem with sandbox games, is that if there is a gallery option that allows you to replay events, then there is really no need for the sandbox.
Once you play through an event, you can just go to the gallery, and replay any event that you want to replay.
The only time that you might need to go into game mode, would be if a new event were added, and after that, it would be available in the gallery.