Remove most bottom space of tabular (or tikz-picture?)

Question

I'm trying to add a table-like structure left of a ganttchart. It appeared to me that labeling every bar with its own tabular of fixed column-width would be the easiest and I got quite close to what I want:

But I can't get rid of the vertical space at the bottom of the table (red circle) - what spacing command does define it?

Any hints are appreciated.

---

If it helps, here is the output of lua-visual-debug

Another discovery, as the gantt chart is actually a tikz picture, I can move it down by:

\tikzset{baseline}

in the preamble, but than I have space above. How to remove all the spacing aorund the tikzpciture?

---

(not so) MWE

\documentclass{scrreprt}

\usepackage[top=25mm, bottom=20mm, left=20mm, right=20mm, foot=20mm]{geometry}
\usepackage{tabularx}
\usepackage{booktabs}
\usepackage{hyperref}


\usepackage{translator}  %// wird zur Übersetung von pgfgantt benötigt.
\usepackage{pgfgantt}

\ganttset{
%vgrid={*{1}{dotted}, draw = none },
vgrid = dotted,
hgrid = dotted,
%hgrid style/.style={thin},
newline shortcut=true,
%
y unit title = 15pt,
title height = 1,
y unit chart = 15pt,
x unit=0.014\textwidth,
%
canvas/.style={
    shape=rectangle, 
    draw=black,
    line width = 0.4pt,
    },
group peaks tip position=0,
group label node/.append style={align=right},
%
bar label node/.append style={align=right},
bar/.append style={fill=gray!20!white},
bar height=0.3,
bar top shift=0.3,
group top shift=0.5,
%
group peaks width=2,
}

\newcommand{\tabgantt}[1]{
\small
\begin{tabular}{p{0.1\textwidth}@{\hskip 0.015\textwidth}p{0.51\textwidth}}
#1
\end{tabular}
}

\newcommand{\tabganttgroup}[3]{
\ganttgroup{
\tabgantt{#1}}{#2}{#3}
}

\newcommand{\tabganttbar}[3]{
\ganttbar{
\tabgantt{#1}}{#2}{#3}
}

\begin{document}

\chapter{title}
\section{section}
\subsection{subsection}
\subsubsection*{subsubsection}\label{par:subsection}
\subparagraph{subparagraph}\label{par:subparagraph}

\begin{table}[h]
\setlength{\tabcolsep}{0pt}
\caption{my gantt chartt}
\label{tab:gantt}
\centering
\begin{tabular}{@{}|c@{}}

\specialrule{0.4pt}{0pt}{-0.4pt}
\specialrule{0.4pt}{30pt}{0pt}
\specialrule{0pt}{0pt}{-30.4pt}

\begin{ganttchart}{1}{24}

\gantttitle[]{2017}{12} \gantttitle[]{2018}{12}  \\
\gantttitle{I}{3}\gantttitle{II}{3}\gantttitle{III}{3}\gantttitle{IV}{3}
\gantttitle{I}{3}\gantttitle{II}{3}\gantttitle{III}{3}\gantttitle{IV}{3} \\

\tabganttgroup{ & \nameref{par:subsection} }{4}{15} \\
\tabganttbar{\ref{par:subsection} & \nameref{par:subparagraph} }{4}{15} \\
\tabganttbar{\ref{par:subsection} & \nameref{par:subparagraph} }{4}{15} \\
\tabganttbar{\ref{par:subsection} & \nameref{par:subparagraph} }{4}{15} \\
\tabganttgroup{ & \nameref{par:subsection} }{4}{15} \\
\tabganttbar{\ref{par:subsection} & \nameref{par:subparagraph} }{4}{15} \\
\tabganttbar{\ref{par:subsection} & \nameref{par:subparagraph} }{4}{15} \\
\tabganttbar{\ref{par:subsection} & \nameref{par:subparagraph} }{4}{15} 
\end{ganttchart}%
\\
\specialrule{0.4pt}{0pt}{-0.4pt}

\end{tabular}
\end{table}
\end{document}