You can definitely use a gravity system at home to run the HOTTAP however please be mindful that you will require a specific set of conditions in order to meet adequate operation of the system.
You'll need 10PSI minimum to open up the pressure switch on the inside of the unit and you'll need 6LPM flow rate as well.
Usually we find that with gravity systems (this is assuming a tank 10-15m above the shower on the side of a hill) has enough pressure to fire up the HOTTAP on the lowest flow setting, but then once you try to adjust the dial to anything more than 1/4 water flow, the unit will cut out.
This is because there isn't enough water flow and pressure to keep that pressure switch inside the unit opened up, so it closes up and the system shuts down.
If the tank you're talking about is anything less than 10m above the shower, then you'll run into issues. If you're not running through the HOTTAP than gravity fed shower is OK, it's just when you want to connect to the HOTTAP you'll need to make sure you have enough pressure/water flow.
Reason for that is the size of the burners are designed for a certain water flow/pressure. If the water flow comes in at a much lower flow rate, then the water will heat up very very quickly way beyond the safety cut-off of 50C (basically it'll be useless as you can't have a shower).
From past experiences we have found in many cases it is easier to obtain a pump to enjoy the system to its full potential. Check out our FLOTAP for further information.