Showers require hot and cold water to be mixed together under pressure. For a shower mixer valve which is designed to work on low water pressure to work efficiently, the shower head must be a minimum of 1000mm below the cold water tank.
To find out if you could use a gravity shower you first put your thumb on the end of the bathroom basin tap with the purpose of stopping the flow of water when you slowly turn the tap on. If you can hold the water back this will indicate that you have low water pressure which is probably tank feed. Find the cold water tank ( usually in the loft) and measure how high the bottom of the tank is above the proposed location of the shower head. If it is around 1000mm it will work but consider using a pump for much better performance.