We investigate the low-temperature electron transport properties of chemically reduced graphene oxide (RGO) sheets with different carbon $s{p}^2$ fractions of 55$\%$ to 80$\%$. We show that in the low-bias (Ohmic) regime, the temperature ($T$) dependent resistance ($R$) of all the devices follow Efros-Shklovskii variable range hopping (ES-VRH) $R\sim \exp \left[{(T_{\mathrm{ES}}/T)}^{1/2}\right]$ with $T_{\mathrm{ES}}$ decreasing from 3.1×10${}^4$ to 0.42×10${}^4$ K and electron localization length increasing from 0.46 to 3.21 nm with increasing $s{p}^2$ fraction. From our data, we predict that for the temperature range used in our study, Mott-VRH may not be observed even at 100$\%$ $s{p}^2$ fraction samples due to residual topological defects and structural disorders. From the localization length, we calculate a band-gap variation of our RGO from 1.43 to 0.21 eV with increasing $s{p}^2$ fraction from 55 to 80$\%$, which agrees remarkably well with theoretical predictions. We also show that, in the high bias non-Ohmic regime at low temperature, the hopping is field driven and the data follow $R\sim \exp \left[{(E_0/E)}^{1/2}\right]$ providing further evidence of ES-VRH.