System.gc() is just a convenience for Runtime.getRuntime().gc().
However, if you are genuinely a beginner, you should never be calling either of these methods.
99.9% of the time, Java will garbage-collect as appropriate and does not need you to tell it to do so. Calling one of the gc() methods does not force garbage collection to happen, either; it only suggests to the JVM that now might be a good time for some garbage collection.
Betty Rubble? Well, I would go with Betty... but I'd be thinking of Wilma.