Hi!
Strictly speaking, you do not need to install any particular plugin to develop JAX-WS web services. I imagine the WTP tools are helpful, because you will probably want to create a Dynamic Web Project in Eclipse when developing a web service. The simplest form of a JAX-WS web service is a @WebService annotated
Java class, nothing more is required - no deployment descriptors etc. The (dynamic web) project in which the class resides is then simply deployed to, for instance, GlassFish.
Regretfully, I don't know about Eclipse Ganymede and WTP - I guess it may depend on the Eclipse package (for instance, if it was the JavaEE package or the JavaSE package).
Best wishes!