Is it possible to make a day trip to Gubei and Simatai at this time of year, using public transport.

Yes. You can take subway line 2 or line 13 to Dongzhimen Station first, then take bus 980 from Dongzhimen Transit Hub to Miyun Bus Station and then change to bus Mi 37, Mi 50, or Mi 51 to Simatai Village Station. There are also direct tourist buses from Dongzhimen Transit Hub to Gubei Water Town and the bus is available at 9:00, 12:00 and 15:30.