Wouldn't it be easier to filter outbound packets destined for Gmail's SMTP servers and prevent Chinese email users from sending email to Gmail users? (This is an honest question.)
I think you mean Google's MX servers (the remote end when sending email through SMTP). If so, in a word, "yes", much easier, assuming users are using SMTP servers based in China. Indeed, you could simply prevent the connections from ever taking place, simulating authentication errors.
Since SMTP allows forwarding by other servers this would require deep packet inspection.
If you mean the SMTP protocol supports chained delivery routes, then I do not think this is true (at least not used in practice). However, business customers of Gmail (at least) can request that a different SMTP server than Google's be used for outgoing mail, and (of course) anyone using an external mail client can send using any SMTP server they like.
From the summary:
HGST decided to go the helium route, allowing it to pack more platters into a drive