We present Gemini near-infrared spectroscopic observations of six luminous quasars at z=5.8$sim$6.3. Five of them were observed using Gemini-South/GNIRS, which provides a simultaneous wavelength coverage of 0.9--2.5 $mu$m in cross dispersion mode. The other source was observed in K band with Gemini-North/NIRI. We calculate line strengths for all detected emission lines and use their ratios to estimate gas metallicity in the broad-line regions of the quasars. The metallicity is found to be supersolar with a typical value of $sim$4 Z_{sun}, and a comparison with low-redshift observations shows no strong evolution in metallicity up to z$sim$6. The FeII/MgII ratio of the quasars is 4.9+/-1.4, consistent with low-redshift measurements. We estimate central BH masses of 10^9 to 10^{10} M_{sun} and Eddington luminosity ratios of order unity. We identify two MgII $lambdalambda$2796,2803 absorbers with rest equivalent width W_0^{lambda2796}>1 AA at 2.2<z<3 and three MgII absorbers with W_0^{lambda2796}>1.5 AA at z>3 in the spectra, with the two most distant absorbers at z=4.8668 and 4.8823, respectively. The redshift number densities (dN/dz) of MgII absorbers with W_0^{lambda2796}>1.5 AA are consistent with no cosmic evolution up to z>4.
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