Graphdiyne (GDY), a two-dimensional (2D) carbon, uniquely possesses mixed sp–sp2 hybridization, uniform nano-sized porous structure, semiconducting character, and excellent electrical conductivity. These features beneficially promote its applications in many fields, especially gas sensing. Based on density functional theory (DFT) and statistical thermodynamics, this study reports the sensing capabilities of pristine and selected transition metal (i.e., Fe, Sc, and Ti)-decorated GDY to detect environmentally hazardous arsine (AsH3) and phosphide (PH3) gases. We discover that Fe-doped GDY is a high-performance sensing material for detecting AsH3 and PH3 because of its selectivity and ultra-high sensitivity at the part-per-million (ppm) level. The presence of these gases induces measurably drastic changes in the electronic properties of Fe-doped GDY. The promising detection capabilities are fundamentally rooted in the appropriate chemical binding energies (i.e., ranging from -0.80 to -1.80 eV), which are basically rooted in the prominent orbital overlap among Fe-3d and As(P)-4p states. This study has raised the need to design efficient nanosensors using GDY-based materials.