On Instagram, GC stands for "Group Chat". A group chat is a feature on Instagram that allows users to send messages to two or more people at once. Users can create a new group chat by tapping on the "Messages" icon at the top right corner of the screen, then tapping on the pencil and paper icon at the top right corner of the screen, and finally selecting the people they want to add to the group chat. Users can also add more people to an existing group chat that already includes three or more people. Instagram group chats can include up to 250 people.