GAP (Generic Access Profile), which is used to control device connections and broadcast.
GAP makes your device visible to other devices and determines whether your device can or interact with contract devices.
GATT uses the ATT (Attribute Protocol) protocol. The ATT protocol stores the data corresponding to Service and Characteristic in a query table. The query table uses the 16bit ID as the index of each item. Once the two devices establish a connection, the GATT begins. It works, which means that you have to complete the previous GAP protocol.