Fix DMARC / DKIM / SPF problems on a domain

Some emails are being marked as SPAM by Gmail. I'm sending via SendGrid, with non-automated security. I can't remember how I set it up. It was working fine and I'm not sure what's changed but some emails are now having the characters "[SPAM] " prepended to the subject line by Gmail. There are two domains I'd like to have set up with SendGrid. I'm looking to have the first domain done in the next few days and the second domain done a few days after I get the DNS credentials on Monday.