Why does using \biggl <stuff> \biggl not throw an error?

I'd prefer a syntax à la mathtools, where you just specify the size with \big \Big \bigg or \Bigg. In any case you should specify the type of the objects you build, because \textcolor hides it.

\NewDocumentCommand{\virtualparens}{ O{} m }{%
  \mathopen{\textcolor{lightgray}{#1(}}% opening fence
  #2% contents
  \mathclose{\textcolor{lightgray}{#1)}}% closing fence
}

Here's the full example, with comparisons.

\documentclass[twocolumn]{article}
\usepackage{amsmath}
\usepackage{xcolor}

\NewDocumentCommand{\virtualparens}{ O{} m }{%
  \mathopen{\textcolor{lightgray}{#1(}}#2\mathclose{\textcolor{lightgray}{#1)}}%
}

%%% for comparison
\NewDocumentCommand{\wrongvirtualparens}{ O{} m }{% your definition
  \textcolor{lightgray}{#1(}#2\textcolor{lightgray}{#1)}%
}
\NewDocumentCommand{\alsowrongvirtualparens}{ O{} m }{% Steven's definition
  \textcolor{lightgray}{\csname#1l\endcsname(}#2%
  \textcolor{lightgray}{\csname#1r\endcsname)}%
}
%%%

\begin{document}

\begin{gather}
  \sin\wrongvirtualparens[\bigg]{-G\frac{m_1 m_2}{r}} \\
  \sin\alsowrongvirtualparens[bigg]{-G\frac{m_1 m_2}{r}} \\
  \sin\virtualparens[\bigg]{-G\frac{m_1 m_2}{r}} \\
  \sin\biggl(-G\frac{m_1 m_2}{r}\biggr)
\end{gather}

\end{document}

Do you see the difference?

You want to use l version on the left and r version on the right. I modified your syntax a bit to achieve it. Using two left versions does not throw an error, but it could mess up the natural spacings between surrounding material.

However, there is a drawback to using this paired approach...you can't naturally break up the argument across a linebreak, which one can do if he individually specifies the \biggl( and \biggr). So, you have to weigh the advantages and disadvantages.

\documentclass{article}
\usepackage{amsmath}
\usepackage{xcolor}

\NewDocumentCommand{\virtualparens}{ O{} m }{%
  \textcolor{lightgray}{\csname#1l\endcsname(}#2
  \textcolor{lightgray}{\csname#1r\endcsname)}
}%

\begin{document}
\[
  \virtualparens[bigg]{-G\frac{m_1 m_2}{r}}
\]
\end{document}

You can keep the decision about parentheses size to \left...\right primitives:

\def\virtualparens#1#2#3{\mathopen{}\color{lightgray}\left#1
   \color{black}#2\color{lightgray}\right#3\color{black}\mathclose{}}

$$
  \sin \virtualparens ({-G {m_1 m_2\over r}}) x
$$
compare with:
$$
  \sin \Bigl( -G {m_1 m_2\over r} \Bigr) x
$$