Is it possible to visit Simatai and Gubei in winter (Feb。 2017) in a day?

One day is enough to cover them, but you can only have a general look. A single journey takes 3-4h from downtown to the scenic area by public transportation. I suggest you hire a car for a round trip or catch the tourist bus operated by Beijing Tour Dispatch Center, which leaves at 08:30 at Qianmen and leads to the wall/water town directly. But I heard you need to book the wall visiting by yourself.
This is just for your reference!

To fully enjoy yourself, I suggest you see the wall on the first day; stay in the water town for a night and tour around in the next day.
Have fun!