\documentclass[12pt]{book}
\usepackage{graphicx}
\usepackage[hmargin=20mm, vmargin=5mm]{geometry}
\geometry{a5paper}
\RequirePackage{fontspec}
	\defaultfontfeatures{Ligatures={TeX},Numbers={OldStyle}}
	\setmainfont{Minion Pro}
	\newfontfamily{\Initialfont}{Acorn Initials}
\usepackage[autocompile]{gregoriotex}
\RequirePackage{luatex85}

\begin{document}
\gresetinitiallines{2}%
%\grechangestyle{initial}{\Initialfont\fontsize{124}{124}\selectfont}%
\grechangestyle{biginitial}{\Initialfont\fontsize{124}{124}\selectfont}
\gregorioscore{a--ecce-advenit}%
\end{document}