Nobody has mentioned the rather essential sunglasses
even in the depths of winter you'll need a pair, especially as the sun starts getting lower earlier in the day.
Keep a couple of pairs in the bag and get a neck strap so you can hang them around your neck, preferably not too expensive ones as you're guaranteed to leave them in the glider / bus / under a buggy wheel etc etc.
Winter gliding fashion: Airfields are muddy, cold, wet, windy places in winter! Thick jacket, fleece jumper, thermal t-shirt and leggings and outdoor trousers, long/thick socks and boots. I swap the thick jacket for a thin windproof one when flying, and if it's not too muddy, boots for trainers (but remember you have to walk to the glider, and when you bring it back). Warm hat and gloves essential, waterproof if snowy/dewy. You will have to kneel on the grass / pick up muddy ropes. I don't wear gloves in the cockpit, but if you do, wear thin ones.
What gliders does your club have? I find the ASK-13 has room in the cockpit for big boots, as long as they don't interfere with your control, but scrape mud off first (ask the instructor to do a loop to show you why). It's a bit tighter in the ASK-21 though, your club won't be happy if you get mud all over the seats.
Gliders aren't very well sealed, and in K13s it seems that cold air blows straight up your legs, so hope the fashion police aren't about and tuck those trousers into your socks!
Don't get too bulky with clothing when you're in the cockpit, you'll want to move freely - but remember when you land you'll be walking the glider back to the launch point, could be a 5 minute walk and if it's bitterly cold you'll wish you had that extra fleece/windproof jacket!
Post again in Spring for the Summer collection
Good luck!