In pre-Roman times, the inhabitants of Gascony were the Aquitanians (Latin: Aquitani), who spoke a language related to the Old Basque language (which predates the modern Basque language). The Aquitanians inhabited a territory limited to the north and east by the Garonne River, to the south by the Pyrénées mountain range, and to the west by the Atlantic Ocean. The Romans called this territory Aquitania, either from the Latin word aqua (meaning "water"), in reference to the many rivers flowing from the Pyrénées through the area, or from the name of the Aquitanian Ausci tribe (whose name seems related to the Basque root eusk- meaning "Basque"), in which case Aquitania would mean "land of the Ausci". In the 50s BC, Aquitania was conquered by lieutenants of C. Julius Caesar and became part of the Roman Empire.
Later, in 27 BC, during the reign of Emperor Augustus, the province of Gallia Aquitania was created. Gallia Aquitania was far larger than the original Aquitania, as it extended north of the Garonne River, in fact all the way north to the Loire River, thus including the Celtic Gallic people that inhabited the regions between the Garonne and the Loire rivers. These Gallic people (with their Gaulish language) were quite different from the non-Indo-European Aquitanians. This was a deliberate policy of Rome, which sought to gather people from different ethnic background into a single province, in order the avoid the development of a regional identity. In 297, as Emperor Diocletian reformed the administrative structures of the Roman Empire, long claims of the now Romanized descendants of the Aquitanians, who had long desired to be separated from the now also Romanized descendants of the Gallic people inhabiting north of the Garonne, were finally heard and Gallia Aquitania was split into three provinces. The territory south of the Garonne River, corresponding to the original Aquitania, was made a province called Novempopulana (that is, "land of the nine tribes"), while the part of Gallia Aquitania north of the Garonne became the province of Aquitanica I and the province of Aquitanica II. The territory of Novempopulana corresponded quite well to what we call now Gascony. From 297 on, the name "Aquitaine" was never used again for Gascony, despite it having been its original name, and instead became used only for territories north of the Garonne River.
Novempopulana suffered like the rest of the Western Roman Empire from the invasions of Germanic tribes, most notably the Vandals in 407-409. In 416-418, Novempopulana was delivered to the Visigoths as their federate settlement lands and became part of the Visigoth kingdom of Toulouse. The Visigoths were defeated by the Franks in 507, and fled into Spain. Novempopulana then became part of the Frankish Kingdom like the rest of southern France. However, Novempopulana was far away from the home base of the Franks in northern France, and was only very loosely controlled by the Franks.
It is then, around 600, that taking advantage of the power vacuum thus created, the Basque clans descended from their refuge in the western Pyrénées and established their hegemony over Novempopulana. This is why Novempopulana became known as Vasconia (that is, "land of the Vascones", the Latin word "Vasco" later evolving into the word "Basque"). The word Vasconia evolved into Wasconia, and then into Gasconia (w- often evolved into g- under the influence of Romance languages, cf. warrantee and guarantee, William and Guillaume). Although the Basque clans dominated Gascony, the gradual abandonment of the Basque related Aquitanian language in favor of a local vulgar Latin, which was well under its way, was not reversed. This local vulgar Latin later evolved into Gascon. However, Gascon was heavily influenced by the original Aquitanian language (for example, Latin f- became h-, cf. Latin fortia, French force, Spanish fuerza, Occitan fòrça, but Gascon hòrça).