If no MetricSet is configured for the GMetrics Ant Task then it uses the set of Metrics defined by the org.gmetrics.metricset.DefaultMetricSet class. That includes the following Metrics:
Cyclomatic Complexity - implemented by org.gmetrics.metric.cyclomatic.CyclomaticComplexityMetric
Method Line Count - implemented by org.gmetrics.metric.linecount.MethodLineCountMetric
Class Line Count - implemented by org.gmetrics.metric.linecount.ClassLineCountMetric