By controls, I'm guessing you mean control valves, not necessarily control sequence or logic. To properly defrost a freezer evaporator you will need sufficient hot gas, as veekrish has stated. Providing you don't try to defrost more than 1/3 of your total system capacity, you should have enough gas, which is the case with many ammonia refrigeration systems.
You must define "gravity feed". If this is a flooded fan coil, then there are two important design elements required to properly defrost the coil. The first is an offset in the horizontal run of the liquid line at the coil to allow for proper draining of the coil during defrost. The second is the sizing of the flooded surge drum. If defrost condensate is relieved into the drum, the drum should be sized to accomodate this volume of liquid. It should also be large enough to prevent liquid carryover due to high velocities at the end of the defrost cycle.
To properly size these lines and valves, the capacity of the coil, operating conditions (Tevap, Tliq), defrost pressure and vessel size are needed. You can consult 1994 ASHRAE Refrigeration Chapter 3 for more information.
chillin