Gtk.TextBuffer.prototype.get_iter_at_offset

function get_iter_at_offset(char_offset:Number):void {
    // Gjs wrapper for gtk_text_buffer_get_iter_at_offset()
}

Initializes iter to a position char_offset chars from the start of the entire buffer. If char_offset is -1 or greater than the number of characters in the buffer, iter is initialized to the end iterator, the iterator one past the last valid character in the buffer.