File geth-classic-v6.0.8-linux.tar.gz seems to be incorrectly compressed (gzipped twice). Hence, tar fails.
wget https://github.com/ethereumproject/go-ethereum/releases/download/v6.0.8/geth-classic-v6.0.8-linux.tar.gz
tar -xvzf geth-classic-v6.0.8-linux.tar.gz
tar: This does not look like a tar archive
tar: Skipping to next header
tar: Exiting with failure status due to previous errors
file geth-classic-v6.0.8-linux.tar.gz
geth-classic-v6.0.8-linux.tar.gz: gzip compressed data, from FAT filesystem (MS-DOS, OS/2, NT), original size 46593289
gunzip geth-classic-v6.0.8-linux.tar.gz
file geth-classic-v6.0.8-linux.tar
geth-classic-v6.0.8-linux.tar: gzip compressed data, last modified: Tue Aug 13 12:04:32 2019, from Unix, original size 96665600
File geth-classic-v6.0.8-linux.tar.gz seems to be incorrectly compressed (gzipped twice). Hence, tar fails.