Gakkimin's Kamiwaza ability is to manipulate musical instruments and play music. As a Bugmin, he can make music for no reason without anyone noticing.
In the series
Gakkimin was one of the Promins who got captured as Bugmins and debugged by Wanda prior to the beginning of the series.
In episode 9, Gakkimin was one of the Promins who got kidnapped by Bug-Liftmin. He got rebugged in episode 10. Bug-Gakkimin messed around in the old man's sweet shop along with Bug-Micmin after Gigaga delivered them to the old man. Later, he got recaptured and debugged by Yuto who was also capturing and debugging Bug-Liftmin.
Gakkimin fused with Turbomin throughout the Kamiwaza Shaker to become Tur-kimin in episode 30 when he, Yuto and Wanda perform some music together in order to comfront Bug-Motemin.