This paper presents the design and analysis of plasmonic optical logic gates monolithically integrated on an indium phosphide (InP) platform using electrically controlled Ge2Sb2Te5 (GST) phase-change material. The proposed heterostructure (InP substrate, InGaAsP core layer, GST layer, and gold electrodes) achieves non-volatile switching in a compact 260-nm active region, exhibiting approximately 30% transmission in the amorphous GST phase and over 80% attenuation in the crystalline phase. Using finite element method optimization, we propose: a ring resonator-based NOT gate operating at 1.527 μm wavelength with 10.2 dB extinction ratio (ER); cascadable NOR gates achieving 19.93 dB ER; and designs for BUFFER and AND gates with ERs of 12.61 dB and 26.40 dB, respectively. These gates offer sub-wavelength compactness, electrical control capability, zero static power consumption, and direct co-integration potential with InP-based lasers and detectors. This platform establishes a scalable pathway toward non-von Neumann photonic processors and optical communication applications.