  We present the synthesis of D0$_{22}$ Mn$_{3 - \delta}$Ga ($\delta$ = 0, 1)
Heusler alloys by Spark Plasma Sintering method. The single phase Mn$_3$Ga
(T$_\mathrm{c}$ $\simeq$ 780 K) is synthesized, while Mn$_2$Ga (T$_\mathrm{c}$
$\simeq$ 710 K) is found to coexist with a near-stoichiometric room temperature
paramagnetic Mn$_9$Ga$_5$~($\approx$ 15 \%) phase due to its lower formation
energy, as confirmed from our density functional theory (DFT) calculations. The
alloys show hard magnetic behavior with large room temperature spontaneous
magnetization m$_s$(80 kOe) = 1.63 (0.83) $\mu_\mathrm{B}$/f.u. and coercivity
H$_\mathrm{c}$ = 4.28 (3.35) kOe for Mn$_3$Ga (Mn$_2$Ga). The magnetic
properties are further investigated till T$_\mathrm{c}$ and the H$_\mathrm{c}$
(T) analysis by Stoner-Wohlfarth model shows the nucleation mechanism for the
magnetization reversal. The experimental results are well supported by DFT
calculations, which reveal that the ground state of D0$_{22}$ Mn$_2$Ga is
achieved by the removal of Mn-atoms from full Heusler Mn$_3$Ga structure in
accordance with half Heusler alloy picture.
