Gila isim means acting crazy, weird or abnormal due to being possessed by jinns or spirit.
As me lah say, isim comes from the word ism which means name in Arabic. In Malay spiritual studies (i.e. ilmu kebatinan) as used in Silat or even deviant religious studies, isim is a study to use mantra that contains God's names (thus the word isim) that are manipulated to call jinns or spirits to possess our bodies so that the user can be invincible or able to heal other people.
Gila isim happen when you are unable to study the skills, or the knowledge is too heavy for you resulting you to react abnormally. It is believed that this is because evil spirits have damaged your mind and thus lead to brain abnormality.
In everyday life, gila isim may be used informally to say that you are damn crazy and nothing to do with jinn. For example, MLP is gila isim already.
Last note, ilmu isim for the purpose of calling jinn using manipulated Names of God (Allah) and Quranic verses is forbidden in Islam.