We present the measured gas-phase metal column densities in 155 sub-damped Ly {alpha} systems (subDLAs) with the aim to investigate the contribution of subDLAs to the chemical evolution of the Universe. The sample was identified within the absorber-blind XQ-100 quasar spectroscopic survey over the redshift range 2.4=<z_abs_=<4.3. Using all available column densities of the ionic species investigated (mainly CIV, SiII, MgII, SiIV, AlII, FeII, CII, and OI; in order of decreasing detection frequency), we estimate the ionization-corrected gas-phase metallicity of each system using Markov chain Monte Carlo techniques to explore a large grid of CLOUDY ionization models. Without accounting for ionization and dust depletion effects, we find that the HI-weighted gas-phase metallicity evolution of subDLAs is consistent with damped Ly {alpha} systems (DLAs). When ionization corrections are included, subDLAs are systematically more metal poor than DLAs (between ~0.5{sigma} and ~3{sigma} significance) by up to ~1.0dex over the redshift range 3=<z_abs_=<4.3. The correlation of gas phase [Si/Fe] with metallicity in subDLAs appears to be consistent with that of DLAs, suggesting that the two classes of absorbers have a similar relative dust depletion pattern. As previously seen for Lyman limit systems, the gas phase [C/O] in subDLAs remains constantly solar for all metallicities indicating that both subDLAs and Lyman limit systems could trace carbon-rich ejecta, potentially in circumgalactic environments.