There is very little public transport in the Gers, so a car is essential for your holiday. St Malo is the best Channel port as the journey from there is 7 hours (estimated with no breaks). The closest airport is Lourdes (Ryanair) although Toulouse (Easyjet, Ryanair and BA) is very accessible, at just about 1.5hours. Bordeaux is slightly longer with a transfer of just under 2.5 hours.
More detailed information will be given on request, depending on your travel plans.