Check opendylan [1], which blends dynamic typing with optional type annotations [2]. There's a paper [3] and a "dead" branch in github [4] with an actual implementation of gradual typing.
[2]: http://dylanfoundry.org/2014/08/28/type-system-overview/
[3]: http://www.itu.dk/~hame/ilc2010.pdf
[4]: https://github.com/hannesm/opendylan/tree/typed-opendylan