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.
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