尝试这个:
\documentclass[a4paper, 11pt]{book}
\usepackage[utf8]{inputenc}
\usepackage{fourier}
\newcommand\varul[2][3]{\mkern#1mu\underline{\mkern-#1mu#2\mkern-#1mu}\mkern#1mu}
\begin{document}
\[a\underline{\mathcal G} a \]%
\[a\varul{\mathcal G}a \]%
\end{document}