Two series of novel phosphors of green-yellow emitting Ca_8−xSr_xMgGd(PO₄)₇:Eu²⁺ and yellow-orange emitting Sr₈MgGd(PO₄)₇:Eu²⁺,yMn²⁺ were successfully synthesized by a solid-state reaction. The crystal structures, photoluminescence (PL), PL excitation (PLE) spectra and decay times were investigated in detail. Upon excitation at 350 nm, the Ca₄Sr₄MgGd(PO₄)₇:Eu²⁺ phosphor showed strong green emission centered at 513 nm, and the Sr₈MgGd(PO₄)₇:Eu²⁺ phosphor showed strong yellow emission centered at 513 nm and 593 nm, and Sr₈MgGd(PO₄)₇:Eu²⁺,Mn²⁺ showed strong orange emission centered at 510 nm and 616 nm. In addition, a standard warm white-light emitting single-phased phosphor of Sr₈MgGd(PO₄)₇:Eu²⁺,Mn²⁺ with chromaticity coordinates (x, y), correlated color temperature (CCT) and Commission Internationale de l'Eclairage (CIE) coordinates of (0.4497, 0.3929), 2705 K, and 89.7 was realized via energy transfer between Eu²⁺ and Mn²⁺. The energy transfer was demonstrated to be a resonant type dipole–quadrupole mechanism. Moreover, the obtained LED device exhibits an excellent color-rendering index (R_a = 97) at a correlated color temperature of 5651 K with the CIE coordinates (0.3290, 0.3410).