I am in the process of migrating existing Gatsby cloud site to Netlify.
On my Gatsby cloud site, there is a Let’s encrypt SSL certificate associated to it. Is it possible to re-use this SSL certificate on my Netlify replicated site (Netlify App) as part of the migration process from Gatsby cloud to Netlify?
You would only be able to use a custom certificate. At Netlify, we also use Let’s Encrypt, when you configure your domain to Netlify you’ll be assigned a Let’s Encrypt SSL certificate (unless you have a custom SSL certificate).
You can follow this Support Guide for configuring DNS: