def METHOD_NAME(dir=None):
    r"""
    Create and clean up the directory for GAP workspaces.
    INPUT:
    - ``dir`` -- the directory where the workspaces should be stored.
      By default, this is ``DOT_SAGE/gap``
    OUTPUT: the actual workspace directory
    EXAMPLES::
        sage: from sage.interfaces.gap_workspace import prepare_workspace_dir
        sage: prepare_workspace_dir()
        '.../gap'
    TESTS::
        sage: prepare_workspace_dir(os.path.join(tmp_dir(), "new"))
        '.../new'
    """
    if dir is None:
        dir = os.path.join(DOT_SAGE, 'gap')
    # Make sure that the workspace directory exists
    try:
        os.makedirs(dir)
    except OSError:
        if not os.path.isdir(dir):
            raise
    else:
        # Directory was created, add a README file
        with open(os.path.join(dir, 'README.txt'), 'w') as f:
            f.write("It is OK to delete all these cache files.  They will be recreated as needed.\n")
    # Delete all gap workspaces that haven't been used in the last
    # week, to avoid needless cruft.  I had an install on sage.math
    # with 90 of these, since I run a lot of different versions of
    # Sage, and it totalled 1.3GB of wasted space!  See trac #4936.
    # We only do this after creating a new workspace, since this cruft
    # issue is only a problem if workspaces get created every so
    # often.  We don't want to have to do this on every startup.
    now = time.time()
    for F in os.listdir(dir):
        if "workspace" in F:
            W = os.path.join(dir, F)
            try:
                age = now - os.path.getatime(W)
                if age >= 604800:    # 1 week in seconds
                    os.unlink(W)
            except OSError:
                # It's not a problem if W doesn't exist, everything
                # else is an error.
                if os.path.exists(W):
                    raise
    return dir