If you want to extract the ‘sent date’ from the pdf to incorporate in the name you are going to have to extract that date from the pdf either using a regex or some other method. The script here might help get you started. It extracts the ‘to’ and ‘from’ recipients’ from a pdf but could easily be modified to extract the date.
The easier way would be to get Apple Mail to access your gmail account and then use the Devonthink Mail extension to do the import.
Frederiko