Does this look like gmail is cracking down on piracy? Extra funny this is just them sending it to their kindle.
Also, Anna’s Archive linked me to a site for getting around this for Kindle but I forget + bookmarked on my laptop. Will edit this if I find it
Does it link to the Google policy that they say that it violates? Are you certain the archive is clean and doesn’t actually contain an executable malware?
If you’re not sure, you could upload the book to VirusTotal and have it scanned.