Question : unTar files

stupid question from beginner.

can uncompress & untar files that end in _tar.gz by first using 'gunzip' & then 'tar -xf filename'

can't figure out how to 'untar' files that end in '_tar.tar'
'tar -xf filename' & all variations i've tried don't work.
suggestions??

Answer : unTar files

try :
       file gftp-0_21_tar.tar
       mv gftp-0_21_tar.tar{,.gz}; gunzip gftp-0_21_tar.tar.gz