Incompleteness does not imply undecidability, but undecidability implies incompleteness, because completeness implies decidability: in a complete system (powerful enough to describe Turing machines), for every TM M and input x, there's a theorem saying that M halts or one that M doesn't halt (because the system is complete). To decide halting, we therefore just need to enumerate all theorems in the system, and we are guaranteed to stop and find the deciding theorem[1].

In fact, when Gödel saw Turing's work and was convinced that Turing indeed captured the general notion of a formal system, he said that the incompleteness theorem was finally satisfactorily proven for all formal systems.

[1]: See http://www.scottaaronson.com/blog/?p=710