At first it was F.lux. Then came Redshift (inspired by F.lux) which also got a GTK version but only for switching it on/off. And now, a Redshift fork called RedshiftGUI finally brings a GUI for customizing everything.
But what are these? All the applications mentioned above serve only one purpose: to change the color temperature of your screen by the time of day, making it easy for your eyes. This is achieved by entering your location and then Redshift (in the case of this post: RedshiftGUI) takes care of the rest.
Like I said, RedshiftGUI makes it very easy to set up and control Redshift: it can automatically fetch your location based on either your IP or ZIP/address, or you can enter the latitude and longitude manually. You can also alter different color temperature settings using the simple GUI provided by this app as well as enable/disable Redshift.
To use RedshiftGUI, simply download it (link at the end of the post) and start it by going to Applications > Accessories > Redshift GUI, then enter your location (select the Lookup IP method and Redshift GUI will find your location automatically) and tweak some settings or just use the default settings (recommended). After setting everything up, check the "Disable auto-adjust" and then uncheck it again - this is like a 'restart' and gets Redshift GUI working right away.
A note: for me, setting "RANDR" for the backend (under the settings) crashed RedshiftGUI so it is probably best not to alter this option (this is not a bug but apparently this happens for those using proprietary Nvidia graphics drivers). But in case you do, to be able to start RedshiftGUI again, run the following command in a terminal: "rm ~/.redshiftgrc".
To make a real use of RedshiftGUI, make sure you add it to your startup applications: go to System > Preferences > Startup Applications, click on "Add", then in the name field enter "RedshiftGUI" and in the command field: "redshiftgui" (both without the quotes).
Download RedshiftGUI (includes Ubuntu .debs and Windows .exe and source packages) | There is also an Arch package