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As has been summarised in the CDR, 
there are many fundamental reasons
for the necessity to understand the gluon distribution
and the gluon-parton interactions deeper than hitherto.
Half of proton's momentum is carried by gluons.
Gluon self-interaction is responsible for the creation of baryonic
mass. In $pp$ scattering at the LHC, the Higgs particle is predominantly
produced by gluon-gluon interactions.
Gluino pair production, as discussed in Section\,\ref{susy},
predominantly proceeds via $gg \rightarrow g \rightarrow \tilde{g} \tilde{g}$
production
and is hugely uncertain at high masses. On the other hand 
the rise of the gluon density
towards low Bjorken $x \lesssim 10^{-5}$ is expected to be tamed
and a new phase of hadronic matter to be discovered, in which
gluons interact non-linearly while $\alpha_s$ is smaller than $1$.

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\begin{figure}[htbp]
\centerline{\includegraphics[clip=,width=1.\textwidth]{voigluW}}
\caption{Relative uncertainty of the gluon distribution at
$Q^2 = 1.9$\,GeV$^2$, as resulting from an NLO QCD fit to
HERA (I) alone (green, outer), HERA and BCDMS (crossed),
HERA and LHC (light blue, crossed)
and the LHeC added (blue, dark).
Left: logarithmic $x$ scale, right: linear $x$ scale. 
}
   \label{fig:voiglu}
\end{figure}
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From the simulations as described one derives a typical
uncertainty for the gluon density to be reduced
to about $3,~1,~5$\,\% at $x=5 \cdot 10^{-6},~0.005,~0.5$,
respectively. These values of Bjorken $x$ mark
the low value for saturation to be discovered, the central rapidity
value for Higgs production and about the high mass
limit for gluino pair
production. Obviously, see Figure\,\ref{fig:voiglu}, the potential
of the LHC for the determination of the gluon density
over $5-6$ orders of magnitude in $x$, simultaneously with
all quark PDFs and $\alpha_s$, is striking. It has to be
compared with the current status
of huge uncertainty on $xg$ at low and high $x$
as is illustrated in Figure\,\ref{fig:graratglu}.
\begin{figure}[htbp]
\begin{center}
\includegraphics[clip=,width=0.49\textwidth]{ratglulog}
\includegraphics[clip=,width=0.49\textwidth]{ratglulin}
\end{center}
\caption{Ratios to MSTW08 of
gluon distribution and uncertainty bands, at
$Q^2 = 1.9$\,GeV$^2$, for most of the available recent
PDF determinations, taken from \cite{AbelleiraFernandez:2012cc}.
Left: logarithmic $x$ scale, right: linear~$x$ scale.
}
   \label{fig:graratglu}
\end{figure}
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