A gravity-fed system is preferable because it will run without the need for electricity (as long as you run the timer and solenoid valves from a battery system). It's really no different from a pump-fed system. You have a specific starting pressure and you adjust the valves by hand*until every plant is getting what it needs.
*The higher the valve is, the more you'll need to open it. With this knowledge you can make rough adjustments to begin with.
Initially you could set it up without ANY electrical units. Just connect pipes and play with it till it's working. Then you could pay someone to fit the control system.
So a simple system would have a pipe running downhill from the tank.
Each level would have a horizontal take-off pipe controlled by a hand-operated valve.
If some plants need more water than others, then those requiring less water would need individual valves OR you could connect two take-off pipes to the main pipe for each level - one adjusted for high flow and one adjusted for low flow.
There's really nothing to it and, even if you don't get it right first time, all it's costing is your time and a few lengths of (re-usable) pipe.