Fonts in math operators not matching in XeLaTeX

Question

Using XeLaTeX I have changed the body and math fonts to Gill Sans. However, the operators defined using DeclareMathOperator still appear in Computer Modern.

How do I edit my configuration to make these operator names use the same font as in math mode? MWE below:

\documentclass[10pt]{beamer}
\usepackage{amsmath}
\usepackage{mathspec}
\usepackage{fontspec}
\setmainfont[Ligatures=TeX,
ItalicFont={Gill Sans Light Italic},
SlantedFont={Gill Sans Light Italic},
BoldFont={Gill Sans},
BoldItalicFont={Gill Sans Italic},
BoldSlantedFont={Gill Sans Italic},
Mapping=tex-text
]{Gill Sans Light}
\newfontfamily\semibold{Gill Sans SemiBold}
\renewcommand\familydefault{\sfdefault}
\usefonttheme{serif}

\setmathfont{Gill Sans Light}

\begin{document}
\begin{frame}
 min $\min f$
\end{frame}
\end{document}