Hmm, that looks good.
My only idea is that your tape drive requires a certain block size.
Consult its documentation!
gzip/gunzip are not aware of block sizes.
Try without compression and redirection:
find . | cpio -ov /dev/st0
find . | cpio --blocksize=0 -ov /dev/st0
blocksize=0 is what your mt status has reported.
Set it to 16 or 20 or 512 or whatever your documentation says.