Yes, thank you @jwds1978 for clarifying this!
The DNS records for your domain on Cloudflare (the one for which you have set up the Email Routing) – that domain – should have a TXT record with the SPF policy. It looks like this (where in this example I have Email Routing set up for my domain on Cloudflare, denverit.com):
In order to authorize the Gmail SMTP Servers, so that you can send emails from your Gmail email account using your Cloudflare-routed email addresses, you need to replace the the Content in this DNS TXT record entry with…
v=spf1 include:_spf.google.com include:_spf.mx.cloudflare.net ~all
Then click the Save button.