  This is the first paper of a series that will examine the options for
embedding supersymmetric orbifold-GUTs into five-dimensional N=2
Yang-Mills-Einstein supergravity theories (YMESGTs). In particular, we focus on
the allowed couplings of charged hypermultiplets in the lowest dimensional reps
of the gauge groups SU(5), SO(10) and E(6). Our results are within the
classification of homogeneous quaternionic scalar manifolds. In the minimal
coupling of a generation of bulk matter hypermultiplets, supergravity requires
the field content of an SO(10) scenario. In the minimal coupling of $n$ bulk
generations of matter and higgs hypermultiplets, supergravity requires the
field content of an E(6) scenario.
  We also discuss the coupling of tensors and non-compact gaugings in 5D
YMESGTs, which can serve as alternative ways to obtain four-dimensional Higgs
sectors. Charged tensor couplings seem to be difficult to work with
phenomenologically since a U(1) gauge factor is always required when they are
present, and it is not clear if tensors can be put in unified multiplets with
other fields, if this is desired. This seems to imply that tensor coulpings in
GUT scenarios may be better suited in higher dimensional settings. The
non-compact gaugings discussed here are simple, and offer a novel unification
scenario in which the supergravity and vector multiplets are connected by gauge
transformations.
  The main points are summarized in tables and the conclusion. Although the
discussion is in the spirit of a "bottom-up" approach, M-theory is taken as a
motivating background.
