I have an FTP server running on a local synology server. It has an external IP address to which I can connect to using an FTP client on port 21. I'm wondering if there is a way to access this FTP server using my domain (ex. ftp.mydomain.com or something similar). The DNS for my domain is managed on Google Cloud DNS.
How do I get started with achieving this?
You can create private or public zone at Google Cloud DNS (depends on your needs) and that's all. In case you need public zone - you should register domain name at first and then use exactly the same NS records you'll get at Google Cloud DNS as DNS servers for your domain at registrar's control panel. Here you can find best practices for private zones. After creating dns zone for your domain you should create A record for your ftp.
If you need more specific information - please update your question.