A sea loch aligned north-south, Gare Loch is 10 kilometres long with an average width of 1.5 kilometres. At its southern end it opens into the Firth of Clyde through the Rhu narrows. The village of Rosneath lies on the western shore just north of Rosneath Point and gives the name Rosneath Peninsula to the whole body of land separating the Gare Loch from Loch Long to the west. The area lies in the Lieutenancy area and former county of Dunbartonshire.
The town of Helensburgh lies on the eastern shore, giving attractive views across to Rosneath Point. Rhu, to the north of Helensburgh, features a yacht marina. Further north the eastern shore is dominated by the Faslane Naval Base, the home of the United Kingdom's Trident nuclear submarines. The loch was the site of a major naval base during World War II and was used to store decommissioned naval vessels in the 1950s. A shipbreakers yard ceased trading in the 1980s and has been absorbed into the naval base.
At the north end of the loch the substantial village of Garelochhead includes adventure holiday facilities and has a station on the West Highland Line. Garelochhead is used as a Royal Marine training centre. The Gare Loch offers good conditions for sightseeing, sailing and sea angling. The Greek sugar boat Captayannis was heading for the sheltered waters of the Loch but now lies wrecked in the Clyde.