The jstat does not support G1 in the JDK6u21.
The result is:
-->/net/vmsqe.russia/export/jdk/6/build/u21/linux-i586/bin/jstat -gc 15288
Warning: Unresolved Symbol: sun.gc.generation.0.space.1.capacity substituted NaN
Warning: Unresolved Symbol: sun.gc.generation.0.space.2.capacity substituted NaN
Warning: Unresolved Symbol: sun.gc.generation.0.space.1.used substituted NaN
Warning: Unresolved Symbol: sun.gc.generation.0.space.2.used substituted NaN
Warning: Unresolved Symbol: sun.gc.generation.0.space.0.capacity substituted NaN
Warning: Unresolved Symbol: sun.gc.generation.0.space.0.used substituted NaN
Warning: Unresolved Symbol: sun.gc.generation.1.space.0.capacity substituted NaN
Warning: Unresolved Symbol: sun.gc.generation.1.space.0.used substituted NaN
Warning: Unresolved Symbol: sun.gc.collector.0.invocations substituted NaN
Warning: Unresolved Symbol: sun.gc.collector.0.time substituted NaN
Warning: Unresolved Symbol: sun.gc.collector.1.invocations substituted NaN
Warning: Unresolved Symbol: sun.gc.collector.1.time substituted NaN
Warning: Unresolved Symbol: sun.gc.collector.0.time substituted NaN
Warning: Unresolved Symbol: sun.gc.collector.1.time substituted NaN
 S0C S1C S0U S1U EC EU OC OU PC PU YGC YGCT FGC FGCT GCT
  ¿ ¿ ¿ ¿ ¿ ¿ ¿ ¿ 16384.0 1944.5 ¿ ¿ ¿ ¿ ¿