The drop gate is used to dump the brass case into a tray to cool off after the anneal process has finished. There are many options for making this gate, but the simplest option is to use a model RC servo and a pushrod to slide a trapdoor out from beneath the case.
Case dropping can also be done using a solenoid. The solenoid chosen should be 12V and have a rated current of <1A.
If you choose to use a model RC servo, almost any servo will work. The rotation angle is defined in the Arduino software and can be tuned if needed. The PCB has a 3 pin servo header that mates with the standard servo plug.