[en] We have measured the cross section for production of ψ and ψ' in bar p and π^- interactions with Be, Cu, and W targets in experiment E537 at Fermilab. The measurements were performed at 125 GeV/c using a forward dimuon spectrometer in a closed geometry configuration. The gluon structure functions of the bar p and π^- have been extracted from the measured dσ/dx_F spectra of the produced ψ's. From the bar pW data we obtain, for bar p, xG(x)=(2.15±0.7)[1-x]^(6.83±0.5) [1+(5.85±0.95)x]. In the π^- case, we obtain, from the W and the Be data separately, xG(x)=(1.49±0.03)[1-x]^(1.98±0.06) (for π^-W), xG(x)=(1.10±0.10)[1-x]^(1.20±0.20) (for π^-Be)