In Java 9, GC logging has been re-implemented with Unified GC logging framework (JEP 271). This initiative is targeted to introduce a common logging system for all components of JVM (compiler, gc, classload, metaspace, svc, jfr, …). As a part of this re-implementation, 43 GC logging flags have been removed. Yes, they are removed, not even deprecated.
JVM will not start if old GC flags are passed. Thus, you are forced to replace these flags in Java 9. Below table summarizes the old Flags and their equivalents in Java 9. Thanks to Poonam Parhar from Oracle for putting together this table.