\documentclass{article}
\usepackage[letterpaper,top=1cm,bottom=1cm,left=4cm,right=4cm]{geometry} 
\usepackage{gregoriotex} % for gregorio score inclusion
\usepackage{CharisSIL}
\begin{document}
\fontsize{12pt}{12pt}\selectfont 
\grechangestaffsize{20}
\gregorioscore[a]{credotest}
\end{document}
