A lot of regular people have made great suggestions... for other regular people. The OP wears shorts at 15º (!!!!) Suggesting baselayer+softshell+hardshell will most likely have him dying of dehydration when he sweats out. And I thought I had a high heat output... wow.
Labgloves, I would stick with the shorts+gaiters combo, but look for any waterproof/breathable pant with a fuzzy tricot lining (not insulation, just fuzzy) and (this is crucial) full side zips from waist to ankle. You can go with something a little lighter that doesn't have internal gaiters - you're already wearing gaiters. I have a great pair of Marmot XCR pants like this, no idea on a model name, I've had them for a couple years already.
They've got the fuzzy lining to take the edge off the cold and wind if it starts getting real ugly out, the full side zips let you go easy-on/easy-off without changing other clothing, taking off crampons, etc, and you won't get too hot. Plus you can just open the side zips from mid-thigh down to mid-calf and get plenty of venting.
If it's cold enough that a pair of GTX pants don't keep you warm enough, switch the shorts to a pair of med. weight tights and pick up any insulated pant, again with full side zips. You can roll with just tights/gaiters during high output, and easily throw on the insulated pant when you slow down or the wind picks up.
This is my system, but for slightly warmer temps.