We present the joint analysis of Neutral Hydrogen (H I) Intensity Mapping observations with three galaxy samples: the Luminous Red Galaxy (LRG) and Emission Line Galaxy (ELG) samples from the eBOSS survey, and the WiggleZ Dark Energy Survey sample. The H I intensity maps are Green Bank Telescope observations of the redshifted 21cm emission on 100 deg(2) covering the redshift range 0.6 < z < 1.0. We process the data by separating and removing the foregrounds present in the radio frequencies with FAsTI ICA. We verify the quality of the foreground separation with mock realizations, and construct a transfer function to correct for the effects of foreground removal on the H I signal. We cross-correlate the cleaned H I data with the galaxy samples and study the overall amplitude as well as the scale dependence of the power spectrum. We also qualitatively compare our findings with the predictions by a semianalytical galaxy evolution simulation. The cross-correlations constrain the quantity Omega(H I)b(H I)r(H I,opt) at an effective scale k(eff), where Omega(H I) is the H I density fraction, b(H I )is the H I bias, and r(H I,opt) the galaxy-hydrogen correlation coefficient, which is dependent on the H I content of the optical galaxy sample. At k(eff) = 0.31 h Mpc(-1) we find Omega(H I)b(H I)r(H I,wig) = [0.58 +/- 0.09 (stat) +/- 0.05 (sys)] x 10(-3) for GBT-WiggleZ, Omega(H I)b(H I)r(H I,ELG) = [0.40 +/- 0.09 (stat) +/- 0.04 (sys)] x 10(-3) for GBT-ELG, and Omega(H I)b(H I)r(H I,LRG) = [0.35 +/- 0.08 (stat) +/- 0.03 (sys)] x 10(-3) for GBT-LRG, at z similar or equal to 0.8. We also report results at k(eff) = 0.24 and k(eff) = 0.48 h Mpc(-1). With little information on H I parameters beyond our local Universe, these are amongst the most precise constraints on neutral hydrogen density fluctuations in an underexplored redshift range.