The gold leaf electroscope have a very thin gold foil attached to the top to a piece of copper. The copper has a large top round called the cap. The whole thing is put inside a glass case, to stop air blowing the delicate gold leaf around.
The piece of copper goes through insulation in the top of the glass case, so that any charge on the gold leaf cannot escape.
Charge can be transferred to the electroscope by wiping the charged object across the cap. The charge flows through the copper and gold, and the gold leaf rises as it becomes repelled by having the same charge as the copper.