Garbage collection is a function of R rather than RStudio. R only periodically does implicit garbage collection, and doesn't routinely release memory that it has asked the operating system to allocate. If you want/need to do this explicitly, see
However, Hadley Wickham has this advice in Advanced R:
Despite what you might have read elsewhere, there's never any need to call gc() yourself. R will automatically run garbage collection whenever it needs more space; if you want to see when that is, call gcinfo(TRUE). The only reason you might want to call gc() is to ask R to return memory to the operating system. However, even that might not have any effect: older versions of Windows had no way for a program to return memory to the OS.
GC takes care of releasing objects that are no longer used. However, you do need to be aware of possible memory leaks.