  Bongartz and Ringel proved that there is no gaps in the sequence of lengths
of indecomposable modules for the finite-dimensional algebras over
algebraically closed fields. The present paper mainly study this "no gaps"
theorem for the bounded derived module category $D^b(A)$ of a gentle algebra
$A$: if there is an indecomposable object in $D^b(A)$ of cohomological length
$l>1$, then there exists an indecomposable with cohomological length $l-1$.
