Untar Without Top-Level Directory

The closest I can find is the --transform option. I don't know for sure, but I'm guessing this is a GNU only flag:

 --transform, --xform EXPRESSION
       use sed replace EXPRESSION to transform file names

Using that, you can pass a sed substitution command and change the / to /abc:

tar cf root.tgz --transform 's/^\//\/abc/' /

This will still print / paths when creating the archive but they will become /abc when you extract it.

Another approach, if your tar does not have the --transform option is to make /abc a symlink pointing to / and then tar that:

$ ln -s / /abc
$ tar chf root.tgz /abc

The -h is essential as that is what tells tar to follow links:

 -h, --dereference
       follow symlinks; archive and dump the files they point to

That will have the effect of setting /abc as the top level directory.

I found three solutions:

1. With GNU tar, using the awesome -I option:

   tar -I pigz -xvf /path/to/archive.tar.gz -C /where/to/unpack/it/
   

2. With a lot of Linux piping (a "geek way"):

   unpigz < /path/to/archive.tar.gz | tar -xvC /where/to/unpack/it/
   

3. More portable (to other tar implementations):

   unpigz < /path/to/archive.tar.gz | (cd /where/to/unpack/it/ && tar xvf -)
   

(You can also replace tar xvf - with pax -r to make it POSIX-compliant, though not necessarily more portable on Linux-based systems.)

Credits go to @PSkocik for a proper direction, @Stéphane Chazelas for the 3rd variant and to the author of this answer.