A few newer bikes in our road club are running 11-28 and some riders dont like the gaps . As for Alpine rides , unless you are doing multiple passes on the same day you should be oK with a 34/28.
The climbs only steepen on the hairpins and mostly the gradients are 8 or 9% . They just go on for hours .
Try and spend a few hours hill repping before you go , or deliberatly choose a lumpy route to build some leg strength before you go.
In my limited experience the tdf climbs seemed easier as they had count down boards with distance and gradient . The lesser known ones were tough as no info on how steep or how far was left ( no garmin )