gtk-label-select-region (label start end)

label -- a gtk-label widget
start -- an integer with the start offset, in characters not bytes
end -- an integer with the end offset, in characters not bytes

Selects a range of characters in the label, if the label is selectable. See the gtk-label-selectable function. If the label is not selectable, this function has no effect. If start or end are -1, then the end of the label will be substituted.