\documentclass[12pt]{standalone}
\input{head}
\begin{document}
\begin{tikzpicture}\selectlanguage{english}
  \path (0,2) node(IT){$GF$} (2,2) node(TT){$(GF)^2$} (4,2) node(TI){$GF$};
  \path                      (2,0) node(T){$GF$} node[below]{$\phantom{\mu}$};
  \draw[->] (IT) -- node[above]{$\eta GF$} (TT);
  \draw[->] (TI) -- node[above]{$GF\eta$} (TT);
  \draw[->] (TT) -- node{$G\varepsilon F$} (T);
  \draw[->] (IT) -- node[left]{$1$} (T);
  \draw[->] (TI) -- node[right]{$1$} (T);
\end{tikzpicture}
\end{document}
%%% Local Variables:
%%% mode: latex
%%% TeX-master: t
%%% End: