Gantt chart spacing, time unit change

Question

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}