Your IP is a Digital Ocean IP address listed under UCEPROTECT- Level 3. There is nothing you can do about it until the entire block is delisted. The entire block is blacklisted by gmail. Check the reputation of your Digital Ocean IP here. UCEPROTECT®-Network - Spam Database Query
Under Level 3.
As workaround, until the IP reputation improves, read the last 2 posts how to setup a relay with sendgrid, mailgun or use Amazon maildelivery Oracle Cloud free tier smtp servers, etc. Digitalocean with Sendgrid SMTP Relay