I use xgamma -gamma 0.7 but when I restart the computer I need to write that command again.
from this question link
You can't do it "permanently", but you can redo the setting at every GUI login by putting a .desktop file in $HOME/.config/autostart/.
Look at examples in /etc/xdg/autostart/
You can edit $HOME/.bashrc and add the line below.
xgamma -gamma 0.7
Then each time you login, the command is executed.