Even with the bluster, Mr. Gladstone seems grateful for Zinnia's reassurances and her dedication to the mission. He pauses to consider her question, then tells her that the more Personas she summons and the more she's able to open herself to them, the easier it will become. He recalls learning to summon several different Personas in a single day. It can be overwhelming, but he has no doubt she'll get the hang of it with enough time and practice!