Gantt chart spacing, time unit change

The chart being produced is too wide, I only neede the unit of time every three-six months rather than every month? what is the valid format of the time unit to change this? e.g. i have tried ‘x3 month’ ‘3*month’ etc.many thanks

documentclass{article}
usepackage[utf8]{inputenc}
usepackage{pgfgantt}

begin{document}
centerline{
begin{ganttchart}[
hgrid,
vgrid,
time slot format=isodate-yearmonth,
time slot unit=month]{2020-03}{2023-9}
gantttitlecalendar{year, month} 
ganttbar{Literature Review}{2020-03}{2020-09}
end{ganttchart}}
end{document}