  The band structure and magnetic properties of organic charge-transfer salt
$\lambda$-(BEDSe-TTF)$_2$GaCl$_4$ (BEDSe-TTF:
bis(ethylenediseleno)tetrathiafulvalene; abbreviated as $\lambda$-BEDSe) are
investigated. The reported crystal structure is confirmed using X-ray
diffraction measurements, and the transfer integrals are calculated. The degree
of electron correlation $U/W$ ($U$: on-site Coulomb repulsion, $W$: bandwidth)
of $\lambda$-BEDSe is larger than one and comparable to that of the
isostructural Mott insulator $\lambda$-(ET)$_2$GaCl$_4$ (ET:
bis(ethylenedithio)tetrathiafulvalene, abbreviated as $\lambda$-ET), whereas
the $U/W$ of the superconducting salt $\lambda$-(BETS)$_2$GaCl$_4$ (BETS:
bis(ethylenedithio)tetraselenafulvalene) is smaller than one. $^{13}$C-NMR and
$\mu$SR measurements revealed that $\lambda$-BEDSe undergoes an
antiferromagnetic (AF) ordering below $T_{\rm N} = 22$~K. In the AF state,
discrete $^{13}$C-NMR spectra with a remaining central peak are observed,
indicating the commensurate AF spin structure also observed in $\lambda$-ET.
The similarity between the structural and magnetic properties of
$\lambda$-BEDSe and $\lambda$-ET suggests that both salts are in the same
electronic phase, i.e., the physical properties of $\lambda$-BEDSe can be
understood by the universal phase diagram of bandwidth-controlled
$\lambda$-type organic conductors obtained by donor molecule substitution.
