Caesar's Commentaries on the Gallic War are his account of his campaigns of 58-50 BC. The apparently objective, third person style of the commentaries probably conceals a propaganda purpose - shoring up Caesar's position in the political struggles at Rome which would lead to the conflict with Pompey and the Senate recorded in his later Commentaries on the Civil War.
The Commentaries are notably as a description of Celtic Gaul, and an early account of much of Western Europe. Caesar's campaigns took him across the Rhine into Germany and across the channel into Britain, and often involved diplomatic alliance-building as much as fighting, at least until the general revolt of the Gauls under Vercingetorix which ended with the climactic siege of Alesia. An eighth book, by Caesar's lieutenant, the later consul Aulus Hirtius, covers the period between Alesia and the Civil War.
The Gallic War is often the first authentic text used in introducing students to Latin, much as Xenophon's Anabasis is for Ancient Greek. Some of the free resources below may be useful for that purpose notably the bilingual Loeb and Perseus texts, and the Memrise flashcard app. Dr Butler's 1851 map is still useful for following the narrative.
Free online texts
Gutenberg: "De Bello Gallico" and Other Commentaries by Julius Caesar, translated by Thomas De Quincey. Multiple formats.
Gutenberg: C. Iuli Caesaris De Bello Gallico by Julius Caesar. Latin text. Multiple formats.
Internet Classics Archive: The Gallic Wars, by Julius Caesar, translated by W. A. McDevitte and W. S. Bohn. HTML and text formats.
Latin Library: C. IVLI CAESARIS COMMENTARIORVM DE BELLO GALLICO LIBER PRIMVS. Latin text. HTML format.
Wikisource: Commentaries on the Gallic War, Online text.
BBC Radio Great Lives: Julius Caesar. Matthew Parris with Barry Cunliffe.
BBC Radio 4 In Our Time: Julius Caesar. Melvyn Bragg with Christopher Pelling, Catherine Steel and Maria Wyke.
Dickinson College Commentaries: Selections from the Gallic War - Latin text, notes, vocabulary, and media for selections from The Gallic War by Julius Caesar, intended for readers of Latin.
Librivox: Commentaries on the Gallic War - public domain audiobook.
Memrise: AP Latin Caesar, by billclausen. One of a number of relevant vocabulary courses on the site.
Wikipedia: Commentarii de Bello Gallico
The Great Conversation: Further reading at Tom's Learning Notes
Caesar: Commentaries on the Civil War.
Caesar: The Alexandrian War.
Caesar: The African War.
Caesar: The Spanish War.
Plutarch: Life of Caesar in the Parallel Lives.
Suetonius: Life of Caesar in Lives of the Twelve Caesars.
Samuel Butler: Atlas of Ancient Geography (1841). Particularly: Britannia (3.7 MB) | Gallia (France) (4.0 MB) | Germany (3.4 MB) | Italy, North (4.1 MB) | Vindelicia, Rhaetia, Noricum, Pannonia, and Illyricum (3.0 MB).
Latin Resources: Online materials for learning Latin.