A quantitative agreement between nitrile relative abundances and Titan's atmospheric composition was recently shown with a reactor simulating the global chemistry occurring in Titan's atmosphere [Gautier et al. (2011) Icarus, 213: 625]. Here we present a complementary study on the same reactor using an in-situ diagnostic of the gas phase composition. Various initial N₂-CH₄ gas mixtures (methane varying from 1 to 10%) are studied, with a monitoring of the methane consumption and of the stable gas neutrals by in-situ mass spectrometry. Atomic hydrogen is also measured by optical emission spectroscopy. A positive correlation is found between atomic hydrogen abundance and the inhibition function for aerosol production. This confirms the suspected role of hydrogen as an inhibitor of heterogeneous organic growth processes, as found in [Sciamma-O'Brien et al. (2010) Icarus, 209, 704]. The study of the gas phase organic products is focussed on its evolution with the initial methane amount [CH₄]₀ and its comparison with the aerosol production efficiency. We identify a change in the stationary gas phase composition for intermediate methane amounts: below [CH₄]₀=5%, the gas phase composition is mainly dominated by Nitrogen-containing species, whereas hydrocarbons are massively produced for [CH₄]₀>5%. This predominance of N-containing species at lower initial methane amount, compared with the maximum gas-to solid conversion observed in Sciamma-O'Brien et al. 2010 for identical methane amounts confirms the central role played by N-containing gas-phase compounds to produce tholins. Moreover, two protonated imines (methanimine CH₂=NH and ethanamine CH₃CH₂=NH) are detected in the ion composition in agreement with Titan's INMS measurements, and reinforcing the suspected role of these chemical species on aerosol production.