It is proved that any almost tilting module over a gentle algebra is partial tilting, that is, it can be completed as a tilting module. Furthermore, it has at most $2n$ complements, which confirms a （deformed）conjecture of Happel for the case of gentle algebras. At the same time, for any $n\geq 3$ and $1\leq m \leq n-2$, there always exists a connected gentle algebra with rank $n$ and a pre-tilting module over it with rank $m$ which is not partial tilting. The tool we use is the surface model associated with the module category of a gentle algebra. The main technique is doing inductions by cutting the surface, which is expected to be useful elsewhere.