The buses in Gubeikou have boots in the back or under the vehicle, so don't worry about the safety of your luggage, but plz don't carry too much luggage as it will become a heavy burden when you climb the wall. Good trip!

Thank you for your answer, but I was actually thinking about leaving our luggage in some luggage depot in Gubeikou bus station... but is there a good bus station in Gubeikou with such a depot?

And is it possible to check online for the timetables of the buses fromn Chengde to Gubeikou and from Gubeikou to Beijing?

Generally speaking, in China, all big bus stations have depots, but Gubeikou bus station is a small station, so I'm not sure whether it has a depot or not. For safety concern, you can leave your luggage in the hotel. The online timetable of the bus from Chengde to Gubeikou is not available at present.