How to draw an arc in Tikz that has gaps specifically placed in it

According to the pgfmanual section 15.3.2 Graphic Parameters: Dash Pattern, you have several possibilities:

\documentclass[tikz,border=10pt]{standalone}

\begin{document}
\begin{tikzpicture}[very thick]
\draw[dashed] (6,2.5) arc (0:180:4cm and 1cm);
\end{tikzpicture}

\begin{tikzpicture}[very thick]
\draw[loosely dashed] (6,2.5) arc (0:180:4cm and 1cm);
\end{tikzpicture}

\begin{tikzpicture}[very thick]
\draw[dash pattern=on 6mm off 7mm] (6,2.5) arc (0:180:4cm and 1cm);
\end{tikzpicture}
\end{document}

The result:

Try the dash pattern style on your path with on and off pairs:

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
\draw [dash pattern=on 1.5cm off 1cm on 1cm off 1cm on 1cm off .5cm on 1cm off .5cm]
    (6,2.5) arc (0:180:4cm and 1cm);
\end{tikzpicture}
\end{document}