  Let $G$ be a connected reductive group over a $p$-adic local field $F$. We
propose and study the notions of $G$-$\varphi$-modules and
$G$-$(\varphi,\nabla)$-modules over the Robba ring, which are exact faithful
$F$-linear tensor functors from the category of $G$-representations on
finite-dimensional $F$-vector spaces to the categories of $\varphi$-modules and
$(\varphi,\nabla)$-modules over the Robba ring, respectively, commuting with
the respective fiber functors. We study Kedlaya's slope filtration theorem in
this context, and show that $G$-$(\varphi,\nabla)$-modules over the Robba ring
are "$G$-quasi-unipotent", which is a generalization of the $p$-adic local
monodromy theorem proven independently by Y. Andr\'e, K. S. Kedlaya, and Z.
Mebkhout.
