Cleaning the .git directory with gc

I've recently started using git, and while experimenting with git commit --amend, I noticed that git gc does not do what I expected. Example:

  $ mkdir foo && cd foo
  $ git init
  $ dd if=/dev/urandom bs=1k count=1000 of=rand.dat
  $ git add .
  $ git commit -a -m 'first rev'
  $ du -s .git
  1100    .git

1 MB file checked in, 1 MB repository.  So far, so good.

  $ dd if=/dev/urandom bs=1k count=1000 of=rand.dat
  $ git commit -a -m 'replaced first rev' --amend
  $ du -s .git
  2120    .git

At this point, I expected the --amend command to notice that the amended commit contains a replacement for the old file, and thus that the repository didn't grow. I then figured that if --amend doesn't do that by itself, git gc surely will:

  $ git gc
  $ du -s .git
  2104    .git

So, why doesn't gc remove the data from the first commit? Is it still accessible, even though the log doesn't show it?

Is it possible to actually replace the commit, i.e., to make it exactly like the first commit never happend at all? (Without modifying the repository by hand.)