Recently, newly created unnatural fluorescent nucleobase analogs have gained increasing attention. In the present work, a comprehensive theoretical study on the structural, electronic, and excited-state properties of y-guanine (yG-t1) and its five possible tautomers (yG-t2, yG-t3, yG-t4, yG-t5 and yG-t6) is performed. Tautomerization analysis reveals that the canonical form of yG is not the most stable tautomer in the gas phase since it has three tautomers with the same stabilities. The spectroscopic properties are investigated: It is found that these tautomers have different absorption spectra, and so we can distinguish them by their spectroscopic signatures. In addition, effects of methanol solution and hydrogen bonding with cytosine on the absorption and emission spectra are examined. The methanol solution is found to red-shift both the absorption and emission maxima of the studied bases except for yG-t1, for which the absorption and emission maxima have blue-shifts after solvation. On the other hand, hydrogen bonding with cytosine is found to are blue-shifted both the absorption and emission maxima of yG-t1, yG-t2, yG-t5, and yG-t6. Theoretical predictions here are helpful for the investigation of the tautomerism of yG and the optical properties of yDNA.