Any timeline is fine, but I'm most interested in Frankish or pre-Frankish Gascony. Even something that focuses on a specific major city would be fine. Preferably in English, I guess.
There's some recent short academic articles that might give you the overview you're looking for : there's more to it with several interesting thesis or academic synthesis but I tried not to list them if you're not that comfortable with non-English sources. If I'm mistaken, don't hesitate to tell me and I'll include them.
General overview
This summary mostly focuses on the differentiation and distinction of Gascons regarding both Basque, 'Occitan' and French ensemble, but would provide a first light onto the history of the region in the Early Middle Ages.
An older synthesis, but that would give you a broad view of Gascony history in a long perspective.
Pre-Roman Aquitaine
A recent assessment of the linguistic of pre-Roman and early Roman Aquitaine, stressing the importance of Proto-Aquitain (related to Basque, quite possibly an earlier stage) along the local cohabitation with Gaulish populations.
Roman Aquitaine
This short article gives itself a greater focus on the perception of Aquitain Gaul by Romans, and their relations with the region up to its conquests.
While the synthesis is about the Roman province of Aquitaine in its imperial extension (that is way beyond the Garonne, up to the Loire), it provides both details and context to the region that would become Gascony (that is the 'old' Aquitaine and the later province of Novempopulania/Third Aquitaine) until the collapse of the Roman Empire.
Early Medieval Gascony
Two successive summaries on the emerging regional peoples, their relations with Franks, and the growing distinction of a Gascon ensemble. (Both summaries are roughly divided by the rise of the Carolingian dynasty)
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You might find some short context in Ian Wood's The Merovingian Kingdoms, 450-751 (Chapter 10 : The Merovingians and their neighbours)