We report the measurement of the R3 = [O  III ] λ 5008/H β ratios for 54 galaxies in the GLIMPSE-D survey. Thanks to gravitational lensing, our sample includes galaxies with −20 ≲  M _UV  < −14 at z  = 6 − 9. We derived oxygen abundances using calibrated relationships. We observe a significant decline in R3 values below M _UV  ≳ −18, which we interpret as evidence of decreasing metallicities in fainter regimes. We explored four prescription models of the evolution of R3 with UV emission based on the new measurements and results from previous surveys. Applying these models to the GLIMPSE [O  III ]+H β luminosity functions, we measured and extrapolated the ionising photon-production rate, Ṅ _ion of galaxies down to very faint limits (SFR _{H α}  ≳ 5 × 10 ⁻³ M _⊙ yr ⁻¹ ). If our results can be generalised, they indicate a dominant contribution of star-forming galaxies to re-ionisation and are consistent with the recent discovery of ultra-faint metal-poor galaxies. Our measurements of the relative contribution of each luminosity bin show that galaxies with L _{H α}  ≈ 10 ^{41 − 42} erg s ⁻¹ dominate at 8 <  z  < 9, but the relative contributions become more uniform at 7 <  z  < 8. Extreme models either under- or over-estimate the ionising photon budget, while intermediate models align with recent observational constraints.