In the editor, create a new file and go to the Fav Class Optn tab. Press the "New (Copy)" button on the bottom right, and find "Human Sorcerer" in that list. Write down the Id that's in parentheses (fcSorHuman in this case) on a piece of scrap paper, and double click that option.
Now, on the right-hand side of the screen, find the "Replaces Thing Id" box - enter the Id of the thing you want to replace: fcSorHuman. In the main group of options, find "Available to Races...", and add Gnome to that list.
Now to add this to the Gnome race.
Switch to the Race tab, and use "New (Copy)" to make a copy of the Gnome - again, write down the Id, and set this version to replace the original. Then, among the options, find "Favored Class Options", and choose "Human Sorcerer" among those. Save both of your changes and use the "Test Now!" button at the top of the screen to add them to your data set.