  Structure of weakly bound/unbound light nuclei is often related to the
low-energy decay channels involving composite particles like deuteron or
${\alpha}$-particle. These channels are essential to understand the appearance
of light nuclei in the Big Bang nucleosynthesis or the helium fusion. We
generalize the Gamow shell model (GSM) in coupled-channel (GSM-CC)
representation to include reaction channels with the composite particles. In
the core + valence particle formulation, this unified microscopic approach for
structure and reactions involving weakly bound/unbound nuclei can be also
applied to study low-energy properties of heavy nuclei. As the first
application of this generalized GSM-CC approach, we describe the structure of
6Li and deuteron - ${\alpha}$-particle elastic scattering using the same
effective Furutani-Horiuchi-Tamagaki (FHT) type nucleon-nucleon interaction.
Asymptotically, the deuteron structure including its continuum is described
using the N3LO chiral force. The bulk of the data, including low-energy
spectrum of 6Li, asymptotic normalization coefficients, and angular
differential cross sections are satisfactorily described.
