Bold and italic are only possible if the font has such variants installed

Bold and Italics aren't universal options available on all fonts. They are specific fonts that should be avaible, and instead of treating them as options of the plain font, Gimp treats them for what they are: fonts in their own rights. So you won't find a "bold" option for "SomeFont", but there is likely a "SomeFont Bold" in your font list.