High-resolution (0.02 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) infrared-absorption measurements show resolved gap modes for $^{10}\mathrm{B}_{\mathrm{Ga}}$ and $^{11}\mathrm{B}_{\mathrm{Ga}}$ impurities in GaP at 284.50 and 283.10 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. These frequencies and those from local modes for the same impurities are analyzed by a method using perfect lattice modes calculated by ab initio density-functional methods and Green's-function techniques for treating the modifications introduced by the defects. It is demonstrated that the large linewidth of 0.9 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for the gap modes and their unusual shapes are due to the second-neighbor interactions with mixed $^{69}\mathrm{Ga}$ and $^{71}\mathrm{Ga}$ isotopes. The measured widths of the $^{10}\mathrm{B}_{\mathrm{Ga}}$ and $^{11}\mathrm{B}_{\mathrm{Ga}}$ local modes in GaP (0.6 and 0.9 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) correlate with the host-lattice two-phonon densities at the appropriate frequencies (592.75 and 570.00 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively). \textcopyright{} 1996 The American Physical Society.