As of now, there is no standard GA classifier implemented in Weka. The only thing that GA applied in Weka is in feature selection. There is an extension available though; its called "Genetic Programming Classifier in Weka", but I have not used it before.
Due to my experience, it is better to add weka.jar file to a java project (in Eclipse IDE for example) and implement your own GA code. By adding weka to your java project, you can call any learning algorithm from weka packages.