The calculated distance (air line) between Saint-Gilles-Croix-de-Vie and Brest is approximately 166 miles respectively 268 kilometers.
By car or train, the actual journey to Brest is certainly longer, as only the direct route (as the crow flies) between Saint-Gilles-Croix-de-Vie and Brest has been calculated here.
Air line (approximately)
268 kilometers
144 nautical miles