my own id in GORM

That's a feature of databinding. You don't want submitted data to be able to change managed fields like id and version, so the Map constructor that you're using binds all available properties except those two (it also ignores any value for class, metaClass, and a few others).

So there's a bit of a mismatch here since the value isn't managed by Hibernate/GORM but by you. As you saw the workaround is that you need to create the object in two steps instead of just one.

I can't replicate this problem (used Grails 2.0.RC1). I think it might be as simple as a missing equal sign on your static mapping = { (you just have static mapping {)

Here's the code for a domain object:

class Book {
    String id
    String name

    static mapping = {
        id generator:'assigned'               
    }
}

And inside BootStrap.groovy:

def init = { servletContext ->
    new Book(name:"test",id:"123abc").save(failOnError:true)
}

And it works fine for me. I see the id as 123abc.