We recommend that you use a 12V pump with your header tank or gravity-fed system, regardless of how and where it is positioned.
Pump requirements: 12V pressure pump with 6-12 litres/min and 40-60 PSI.
Technical breakdown
The HOTTAP needs a continuous water flow of 6 litres/min and continuous water pressure of 10 PSI.
Continuous means that it does not fluctuate (i.e., it does not drop). For example, a garden tap supplies water flow/pressure that does not fluctuate. Whereas, a header tank loses flow/pressure as the water level drops.
This, along with a few other variables, make it difficult to reliably recommend a perfect gravity-fed setup. This is why a 12V pump can bridge the gap, and supply a continuous flow/pressure to the HOTTAP.
Will it ever work?
As a rule of thumb, you will need a minimum of 10m of vertical height on your gravity header tank to have enough pressure to operate correctly. Even then it is not quaranteed.
You will also need to consider the minimum inner diameter of your water pipes (if they are too thin, not enough water will flow), how many angles and bends you have (the sharper the angles, and the more angles you have, the slower the flow), and the water level in your tank (as the water level drops, you lose pressure).