Fluorescence and absorption spectra at 530 nm (H-2(11/2)-> I-4(15/2)), 560 nm (S-4(3/2)-> I-4(15/2)), 660 nm (F-4(9/2)-> I-4(15/2)), 980 nm (I-4(11/2)-> I-4(15/2)), 1530 nm (I-4(13/2)-> I-4(15/2)), and 2710 nm (I-4(11/2)-> I-4(13/2)) of Er3+ in Gd3Ga5O12 single-crystal codoped with Pr3+ have been measured. Judd-Ofelt analysis yields the intensity parameters omega(2) = (0.68 +/- 0.03) x 10(-20) cm(2), omega(4) = (0.60 +/- 0.07) x 10(-20) cm(2), and omega(6) = (0.90 +/- 0.17) x 10(-20) cm(2). A comparison with previously reported values of Er3+-only doping case shows that Pr3+-codoping causes slight change of both omega(2) and omega(4), while onefold increase of omega(6). From calculated radiative rates and measured fluorescence spectra, Er3+ emission cross-section spectra were calibrated at first. Then, the absorption cross-section spectra were calculated using McCumber relation. In parallel, the absorption cross-section spectra were also obtained from the measured absorption spectrum, and compared with those obtained from the McCumber relation. The comparison shows that both methods give consistent result of absorption cross-section spectrum. Further comparison with Er3+-only doping case shows that Pr3+-codoping causes considerable change of Er3+ cross-section value. In spectrally mixing regions of Er3+ and Pr3+, Pr3+ emission affects little the determination of Er3+ emission cross-section as Pr3+ fluorescence is much weaker than Er3+ fluorescence due to low Pr3+ concentration.