Not sure if you already figured it out or not, but here's the guide; every locked CG in the gallery that has 
icon while you are pointing it was only be able to unlock by playing "no romance" route.
To do this, you must not accept any dateable characters as a boyfriend (now Macsen and Zhokhar)
You can using [Trailmakers] to find those CG if you don't have enough time or didn't save the choice part.
in the hint, but didn't understand what does it meant to be