Both GEC Thrissur and CUSAT are good institutions and have their own strengths and weaknesses.
CUSAT is an autonomous university spread across 180 acres. The vast campus and multiple streams within it will allow you to network with people from different educational backgrounds. Also, you will get to attend national level fests and events over there. It has better sports facilities than most Engineering colleges in Kerala. It is in Kalamassery which is near Ernakulam and Kochi, so you will get to enjoy the city life as well. There are non-Keralite students as well at CUSAT. However, the Engineering fees over there is much higher than GEC, Thrissur. Also, it has a tough syllabus for Engineering.
GEC Thrissur’s campus is spread across 75 acres. It has a strong alumni network and it is under the direct control of Kerala Government. You will get ample freedom at GEC. Various cultural events and other activities keep happening within the campus. Its syllabus is easier and its fees are way lower than CUSAT.
Both of these have student groups, organizations and communities like IEEE, TinkerHub, ISTE, SAE, NSS, etc. that will offer you participation in extracurricular activities.
Both the institutions have their own perks. To know more about each, I would suggest you talk to their alumni.