Restarting R without quitting R

You could try

  rm( list=ls() )   # to remove all objects in a session 
  gc()              # may return from R to operating system

but sometimes I find it just easier to kill and start a new R session.

There are other things that are different in a restart:

It sets the search list to a default.

It executes startup code from various places.

BTW, I'm not sure what you mean when you say that gc() "may return from 
R to operating system".

Duncan Murdoch