The simple answer is you shouldn't ever need to call gc() - although it probably won't make much difference if you do as it only gives a suggestion to the garbage collector that it may want to run.
It may have been that in the early days of
Java this had some useful function (I really can't remember) but certainly these days the garbage collector(s) are much better at knowing when to run than you are. They also have less of a performance hit than they used to so you don't notice them running so much - I do remember occasions in the early days when you would get noticeable pauses when a garbage collection cycle ran. I guess improvement to garbage collection algorithms, general performance improvements and massive increases in raw computing power have sorted that out.
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