\documentclass{article}
\usepackage[a4paper,landscape,margin=3cm]{geometry}
\usepackage{tikz}
\usetikzlibrary{calc}

% GanttHeader setups some parameters for the rest of the diagram
% #1 Width of the diagram
% #2 Width of the space reserved for task numbers
% #3 Width of the space reserved for task names
% #4 Number of months in the diagram
% In addition to these parameters, the layout of the diagram is influenced
% by keys defined below, such as y, which changes the vertical scale
\def\GanttHeader#1#2#3#4{%
 \pgfmathparse{(#1-#2-#3)/#4}
 \tikzset{y=7mm, task number/.style={left, font=\bfseries},
     task description/.style={text width=#3,  right, draw=none,
           font=\sffamily, xshift=#2,
           minimum height=2em},
     gantt bar/.style={draw=black, fill=blue!30},
     help lines/.style={draw=black!30, dashed},
     x=\pgfmathresult pt
     }
  \def\totalmonths{#4}
  \node (Header) [task description] at (0,0) {\textbf{\large Task Description}};
  \begin{scope}[shift=($(Header.south east)$)]
    \foreach \x in {1,...,#4}
      \node[above] at (\x,0) {\footnotesize\x};
 \end{scope}
}

% This macro adds a task to the diagram
% #1 Number of the task
% #2 Task's name
% #3 Starting date of the task (month's number, can be non-integer)
% #4 Task's duration in months (can be non-integer)
\def\Task#1#2#3#4{%
\node[task number] at ($(Header.west) + (0, -#1)$) {#1};
\node[task description] at (0,-#1) {#2};
\begin{scope}[shift=($(Header.south east)$)]
  \draw (0,-#1) rectangle +(\totalmonths, 1);
  \foreach \x in {1,...,\totalmonths}
    \draw[help lines] (\x,-#1) -- +(0,1);
  \filldraw[gantt bar] ($(#3, -#1+0.2)$) rectangle +(#4,0.6);
\end{scope}
}

% Example
\begin{document}
\thispagestyle{empty}
\begin{tikzpicture}
  \GanttHeader{.8\textwidth}{2ex}{4cm}{15}
  \Task{1}{Requirement analysis}{0}{3}
  \Task{2}{Devise an abstract}{0.5}{1}
  \Task{3}{Develop interfaces}{3}{3}
  \Task{4}{Adapt existing}{4.5}{3}
  \Task{5}{Implement and evaluate}{5}{4.5}
  \Task{6}{Specify Phase II design}{8}{3}
  \Task{7}{Prepare final report}{11}{1}
\end{tikzpicture}
\end{document}