While the individual energy of a single gamma ray can be very high the total energy flux of all of the gamma rays is quite low, many orders of magnitude less than the power available from ordinary sunlight. Also, it's very difficult to convert gamma rays into electrical power efficiently.
Electron Volts are a unit of energy. It's basically the amount of energy that is released from a single electron travelling through an electric potential difference of one volt. Ten billion electron Volts is a lot of energy for a single particle but it is still one billionth of a single Joule.
While the individual energy of a single gamma ray can be very high the total energy flux of all of the gamma rays is quite low, many orders of magnitude less than the power available from ordinary sunlight. Also, it's very difficult to convert gamma rays into electrical power efficiently.