Manually update autogenerated id by GORM

I'm working with class:

 class Account{
      static mapping = {
          id generator: "uuid2"
      }
 }

I try to add instance of account and manually set its id:

new Account(id: accountId).save(flush:true)

but after flush, id of saved object is changing. I'd like to leave default engine of autogenerating id, but I want also to add functionality to add object with specified id. How can I obtain it? Grails 2.4.5 here.

And an error from stacktrace:

Message: identifier of an instance of com.example.Account was altered from x... to y...

I'd modified @Sandeep Poonia (+1) answer and finally found satisfying solution:

  import java.util.UUID

  class Account{
        UUID id

        static mapping = {
            id generator: "assigned"
        }

        def beforeInsert() {
              if(!id){
                    id = UUID.randomUUID().toString()
              }
        }
  }