> Do I need to use a struct with one slot? Any other tips?

You could do something like:

(setq my-list '(gnus-range (1 . 9) 35 120))

(cl-defgeneric seq-length ((sequence (head gnus-range)))
  (let ((l 0))
    (dolist (elt (cdr sequence))
      (cl-incf l (if (consp elt) (1+ (- (cdr elt) (car elt))) 1)))
    l))

(seq-length my-list)