The project seems to be no longer developed. Last version, 0.2.4, released in August 2014, can be still downloaded from GitHub.
RedshiftGUI changes the color temperature of your monitor to better match that of your surrounding. This means that it sets a cooler color temperature during the daytime (due to natural lighting) and sets a warmer temperature at night (due to indoor lighting). Similar to f.lux, this is a GUI port of the redshift project.