The Generic Attributes (GATT) is the name of the interface used to connect to Bluetooth LE devices. The interface has one or more Bluetooth Services, identified by unique ids, that contain Bluetooth Characteristics also identified by ids.
A GATT client scans for devices that are advertising, connects to a chosen server device, discovers the services, discovers characteristics and then reads from, writes to or sets up a connection to receive notifications from the characteristic.
Introduction to GATT
Generic Attribute Profile (GATT)
Chapter 4. GATT (Services and Characteristics)
How GAP and GATT Work
KBA_BT_0102: BLE Basics (master/slave, GATT client/server, data RX/TX)
Reverse Engineering iBeacon and Eddystone Bluetooth GATT Services
Bluetooth low energy Characteristics, a beginner's tutorial
Bluetooth Low Energy Settings for Alexa Gadgets