Yeah, it would make more sense to create the full size encrypted archive first and then split it. Does gpg have an option to output to stdout rather than an output file? If so, then maybe this will work (I'm just guessing right now)...
Code:
tar cfz - foo/ | gpg -c | split --bytes=2000MB - foo.tar.gz.gpg.part
cat foo.tar.gz.gpg.part* | gpg -d | tar xfz -
If this works as I guess, then the first line will:
1) tar foo
2) encrypt the output of tar foo
3) split the output of encrypt the output of tar foo
and the second line will:
1) merge foo.tar.gz.gpg.part*
2) decrypt the merge of foo.tar.gz.gpg.part*
3) untar the decrypted merge of foo.tar.gz.gpg.part*
I think?