Base layer and a jersey but will I need a gillet,? Never really thought about it before but is it worth investing.?
Either a gillet or a packable waterproof/windproof. Going over 1000m it gets cold very easily. Descending for 20 minutes at 40mph you don't want to start shivering.
I was in the Alps early season this year, so on pretty much all rides I had armwarmers and a packable waterproof.
Baselayer is a tricky one, as it's a hard thing to put on/take off.
Yes, and arm warmers and leg or knee warmers.
Also consider a windproof or waterproof if out in big hills.
Even on a hot day take a gillet or windproof jacket for the descents.
climbs will be hot, descents will be cold, and being the alps don't assume it won't snow, rain etc
MTFU
Brassards, jambieres and the morning paper!
Seriously though, you will need something to keep your torso warm coming back down the hill. A windstopper gilet wouldn't go amiss.