There's a thread on here "Please advice GP living outside". You do well to think ahead ,I know I'm thinking about summer, my pigs overheating, flystrike and if my Chaste boys are going to go all Leslie Phillips when they are on holiday and might meet girls .
Your GPs will be what age when you get them?
It got very cold at night last winter, quite a few times their water bowl froze up, so I was glad they were in.
They can if they are dry, insulated and protected.No draughts, make sure the bedding is dry, even if it means a daily change. Ours liked fleecy blankets to cuddle in.
My hogs were in at night since early Dec til last week and out in their pighouse every day.
But at 6.30 am I was putting a fan heater in for 15 minutes while I filled up 3-4 hot water bottles (I kid you not). And a good light is essential for dark mornings.
They came in at 6pm.Their indoor cage is small 4'x2' split into 2 (they get arsey in confined spaces). so need to be seperate at night (or it's Fight Club in our house)
Loads of hay (I use half a big bag per bedchange) in their wooden house in the shed. I know it's considered not done but I put soft barley straw as a thick layer at the back and sides to insulate the box .They don't eat it, you have to make sure they can't stab themselves (this stuff is softer than alot of the hay) but it bulks up more, and they have plenty of hay.
Insulate with loads of newspaper, cardboard, carpet.
This worked well for us , we got our boys last October, they were 1 yo. Boars don't cuddle up, girls tend to.
But our boys needed run around space and we always quick to tell me off if I was late putting them out.