A loop detector is a coil of insulated wire (the 'loop') buried in a saw-cut groove in the driveway, connected to a detector unit in the gate-motor controller. As a metal vehicle drives over the loop, it changes the loop's inductance. The detector senses the change and fires a relay that triggers an open or hold-open signal to the gate motor.
Two loops are common. An 'open loop' on the approach side triggers the gate open. A 'safety loop' under the gate's path holds the gate open while the loop is occupied (so a car parked under the gate can't have the gate close on it).
Loop wire is typically 1.5 mm or 2.5 mm single-core, laid in 3 to 5 turns, then the saw cut is sealed with hot-pour bitumen. Pet-friendly small-vehicle-only detection is fiddly: motorbikes, bicycles, and shopping trolleys are too small to register reliably.