First, we'll need to configure a basic implementation of OpenSMTPd. Start with /etc/mail/secrets, where you'll need to enter your gmail account and password.
echo 'googlemail firstname.lastname@example.org:openbsdgmailpassword' >> /etc/mail/secrets
chown root:_smtpd /etc/mail/secrets
chmod 640 /etc/mail/secrets
Next, you'll need to log into your gmail account and navigate to the following URL and "Turn on" less secure apps.
Google will warn you that the sky is crashing, but honestly, this is a dedicated account strictly for /sending/ mail from your firewall. Right? .....
Edit your /etc/mail/smtpd.conf file and add the following lines:
table secrets db:/etc/mail/secrets.db
accept for any relay via tls+auth://email@example.com:587 auth <secrets>
Now try sending a message, until a route is enabled, it will likely fail. This may take a few minutes, but once you see the 'smtp-out: Enabling route  <->->', it's likely good to go.
echo "Testing mail from t5220." | mail -s "Mail test from T5220" firstname.lastname@example.org
Here's the /var/log/maillog from my test:
May 6 03:28:19 t5220 smtpd: smtp-out: Enabling route  <-> 22.214.171.124 (ob-in-f108.1e100.net)->
May 6 03:28:21 t5220 smtpd: smtp-out: Enabling route  <-> 126.96.36.199 (ob-in-f109.1e100.net)->
May 6 03:29:05 t5220 smtpd: smtp-in: New session 5c9e377d25bc3344 from host t5220.local [local]
May 6 03:29:05 t5220 smtpd: smtp-in: Accepted message 6f338ef3 on session 5c9e377d25bc3344: from=
May 6 03:29:05 t5220 smtpd: smtp-in: Closing session 5c9e377d25bc3344
May 6 03:29:05 t5220 smtpd: smtp-out: Connecting to tls://188.8.131.52:587 (ob-in-f108.1e100.net) on session 5c9e37803c19815b...
May 6 03:29:05 t5220 smtpd: smtp-out: Connected on session 5c9e37803c19815b
May 6 03:29:05 t5220 smtpd: smtp-out: Started TLS on session 5c9e37803c19815b: version=TLSv1/SSLv3, cipher=ECDHE-RSA-AES128-GCM-SHA256, bits=128
May 6 03:29:06 t5220 smtpd: smtp-out: Server certificate verification succeeded on session 5c9e37803c19815b
May 6 03:29:08 t5220 smtpd: relay: Ok for 6f338ef3fa99c038: session=5c9e37803c19815b, from=
May 6 03:29:18 t5220 smtpd: smtp-out: Closing session 5c9e37803c19815b: 1 message sent.
Let me know if you have other providers you use with a configuration like this, I'd be interested to know how yours works.