godan verb (-ru), transitive verb, intransitive verb
show nostalgia for; long for; reminisce fondly about
Describes someone else's visible nostalgia or fond reminiscing; not used for one's own feelings. Often implies the person is visibly missing or talking about the past.
祖母は昔の写真を見て懐かしがっていた。
My grandmother was looking at old photos and reminiscing fondly.
彼
He often talks nostalgically about his school days.