While technically this is possible, the results will be less than ideal. You can use Gmail's "Send As" feature to send a secure email from Gmail that will appear in your recipient’s inbox to have come from your third-party provider address, but the secure policy is still owned by the Gmail address.
This leads to some undesirable behavior. If your recipient uses Virtru, their reply will be sent by their mail provider, which can parse the "reply-to" address (the address you "sent as"), so their reply will be sent back to your third-party address, then be automatically forwarded to Gmail. When you attempt to open it in Gmail, you'll be denied access, because this is not the intended recipient of that email.
However, if your recipient does not have Virtru installed and reads in our Secure Reader, their reply will be sent back to the address that owns the secure policy - the Gmail address - instead of being sent to the reply-to address. You will be able to read their reply in your Gmail inbox, but it will not be delivered at all to your third-party inbox.