The workaround to go from GAS -> FTP is as follows. I've been using this method for a number of years in production.
- From apps script, save the file to google drive
- Using Zapier, trigger to copy new filesfrom the google drive folder to a DropBox folder
- Using Microsoft Flow, trigger to copy new files from the Dropbox folder to an FTP or SFTP location.
You can probably do step 3 in Zapier as well. I use Microsoft Flow because I already had a significant number of automated tasks setup there. For Step, 2, I use Zapier because MS flow does not yet have a built-in trigger available based on a new file being saved to google drive. You would have to make your own custom trigger using the google drive API's.