We present ab initio pseudopotential-density-functional calculations for the electronic structure of the dilute magnetic semiconductor Mn$_x$Ga$_{1-x}$P, with a realistic $x=0.063$, in its ordered ferromagnetic phase. We find that it possesses a spin-polarized valence band that could support ideal spin-polarized hole transport. We further find spin-polarized features in the conduction band that could support ideal spin-polarized transport of minority electrons. As such, it emerges as a silicon-lattice-matched candidate material for spintronics applications.