As long as I know, you don't necessarily need to call garbage  collector explicitly

It is good know that you should never relay on calling
garbage collector exprlictily. So you must know that
methods that java provides are requests and not
demands. The virtual machine will do its best to do
what you ask, but there is no guarantee that it will
comply.
Anyway, you can also try caling GC through Runtime
class. For example:

Runtime r = Runtime.getRuntime();
r.gc(); // this is an alternate to System.gc()

If you want global information (e.g. your total memory or -Xm' are ok?)
call java with "-verbose:gc -Xloggc:/tmp/gc.log" and you will get a lot of
knowledge on how your system  works. Give "-incgc" a try, but most of the
time it sucks!

If you wana detailed information maybe you can use a memory profiler. Also
Java's system library has 'totalMemory()' and 'freeMemory()' witch can
help too.