Decorating an edge of a cycle in tikz

A simple solution with using of node in the border where needed:

\documentclass[border=3mm]{standalone}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
        \draw[fill=gray, rounded corners]
        (0,0) -- (0,1) -- node[circle, draw, fill=white, inner sep=1.5pt] {} (1,1) -- (1,0) -- cycle;
\end{tikzpicture}
\end{document}

enter image description here

Addendum: Instead nodes with selected shapes, you can also use pic which can contain arrows etc. For example:

\documentclass[border=3mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows.meta}

\begin{document}
    \begin{tikzpicture}[
arr/.pic = {\draw[-{Straight Barb[width=3pt]}] (-1pt,0) -- (1pt,0);}
                        ]
        \draw[fill=gray, rounded corners]
        (0,0) -- (0,1) -- pic {arr} (1,1) -- (1,0) -- cycle;
    \end{tikzpicture}
\end{document}

enter image description here


The answer to "Decorating an edge of a cycle in tikz" is to use edge[decorate].

enter image description here

\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{decorations.markings}

\begin{document}  

\begin{tikzpicture}
  \draw[fill=gray, rounded corners, decoration={markings,mark=at position .5 with {\draw[black, fill=white] circle[radius=2pt];}}]
        (0,0) -- (0,1) edge[decorate] (1,1) -- 
        (1,1)  -- (1,0)-- cycle;
\end{tikzpicture}        
\end{document}

Here is an extended comment since there is another post which points out what had been pointed out several times already, e.g. here: you can use a pic for the arrow. I'd like to complement the statement by adding that you need the sloped and allow upside down keys to make that work properly.

enter image description here

\documentclass[tikz,border=3mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows.meta}
\begin{document}
\begin{tikzpicture}[circ/.style={circle, draw, fill=white, inner sep=1.5pt},
    pics/arr/.style={code={\draw[-{Stealth[length=#1,fill=black]}] (-#1/2,0) -- 
    (#1/2,0); }},pics/arr/.default=1ex]
 % https://tex.stackexchange.com/a/430486   
 \draw[fill=gray, rounded corners]
        (0,0) -- (0,1) -- node[circ]{} (1,1) --
        pic[sloped,allow upside down]{arr} (1,0) -- 
        pic[sloped,allow upside down]{arr=1.2ex} cycle;
\end{tikzpicture}
\end{document}