Straintronics is an emerging field enabling novel tuneable functionalities of electronic, optical, magnetic, or spin devices with advances being fuelled by new developments in van der Walls (vdW) heterostructure engineering and materials design. Here we show, using state-of-the-art quantum Monte Carlo (QMC) methods, that a single phosphorene monolayer exhibits outstanding straintronics functionalities due to discovered colossal strain tunability of its semiconducting electronic gap. First, we determine the equilibrium atomic structure that differs appreciably from available bulk phosphorene experimental data. That enables us to precisely analyze the quasiparticle band gaps for any uniaxial (armchair and zigzag) and biaxial strains which we describe by a quadrivariate paraboloid function of lattice and internal structure parameters. Using the fixed-node QMC calculations fitted by analytical formulas we localize the following excited state crossings: (i) between the direct (Γ→Γ) and direct but reordered (Γ→Γ′) excitations that also imply substantial differences of corresponding transport properties; and (ii) between the direct Γ→Γ and indirect Γ→X excitations. Based on this highly accurate many-body treatment, we predict the gauge factor ≈100 meV/% and an unusual behavior with the band gap remaining direct even if strained by several percent. Consequently, we suggest there is a colossal band gap tunability window, larger by an order of magnitude when compared to quintessential straintronic materials such as MoS2. In addition, we ascertain that the ground state deformation energies exhibit an out-of plane negative Poisson's ratio and auxetic behavior.