This was actually an intentional choice not to do that. Setting up forwarding means you may have email continually passing through Google servers you didn't realize was still doing so. By having a hard cut between the two, I know for a fact that any email received at FastMail is not going through my Gmail account, and any email received at Gmail is, and needs to have the contact information updated.

I have a vacation auto-responder permanently on on Gmail as well to notify any individuals who hit my old address where my new address is.