Basque Substrate
The language spoken in Gascony before Roman rule was part of the Basque dialectal continuum (see Aquitanian language); the fact that the word 'Gascon' comes from the Latin root vasco/vasconem, which is the same root that gives us 'Basque', implies that the speakers identified themselves at some point as Basque. There is a proven Basque substrate in the development of Gascon. This explains some of the major differences that exist between Gascon and other Occitan dialects.
A typically Gascon feature that may arise from this substrate is the change from "f" to "h". Where a word originally began with in Latin, such as festa 'party/feast', this sound was weakened to aspirated and then, in some areas, lost altogether; according to the substrate theory, this is due to the Basque dialects' lack of an equivalent /f/ phoneme. Thus we have Gascon hèsta or . A similar change took place in continental Spanish. Thus Latin facere gives Spanish hacer (or, in some parts of southwestern Andalusia, ).
While some linguists deny the plausibility of the Basque substrate theory, it is widely assumed that Basque, the "Circumpyrenean" language (as put by Basque linguist Alfonso Irigoyen and defended by Koldo Mitxelena, 1982), is the underlying language spreading around the Pyrenees onto the banks of the Garonne River, maybe as far east as the Mediterranean in Roman times (niska cited by Joan Coromines as the name of each nymph taking care of the Roman spa Arles de Tech in Rousillon, etc.). Basque gradually eroded across Gascony in the High Middle Ages (Basques from the Val d'Aran cited still circa 1000), with vulgar Latin and Basque interacting and mingling, but eventually with the former replacing the latter north of the east and middle Pyrenees and developing into Gascon.
Note that modern Basque has had lexical influence from Gascon in words like beira ("glass"), polit ("pretty", Gascon polit/polida) to mention but a few. One way for the introduction of Gascon influence into Basque came about through language contact in bordering areas of the Northern Basque Country, acting as adstrate. The other one takes place since the 11th century over the coastal fringe of Gipuzkoa extending from Hondarribia to San Sebastian, where Gascon was spoken up to the early 18th century and often used in formal documents until the 16th, with evidence of its occurrence in Pasaia still in the 1870s. A minor focus of influence was the Way of St James and the establishment of ethnic boroughs in several towns based on the privileges bestowed on the Francs by the Navarrese kings from the 12th until the early 14th century, while the variant spoken and used in written records is mainly Occitan of Toulouse.
Read more about this topic: Nord-gascon Dialect