How to read root format as a source

Hi everyone,
I want to use phase space as a source for my simulation. My phase space is the root format that was created by Gate 9.0. Because Gate can not support dose calculation in regions that are different from than boxes. So I want to use Geant4 to implement my simulation after creating phase space from Gate.
Can anyone help me to introduce the ways to read particles from the phase space as a source in Geant4?
Thank you in advance. Wish you have a nice day.