[shartte]

If they are using GMail for work, the DNS just points to GMails actual server and authenticates using DKIM as well. Google for Work will provide you with the necessary DNS entries to set. Obviously this will not work with their free offering, you'll need to fork over $6/month for this.

[zbuf]

Yes, though this isn't forwarding. The mail is actively being delivered into gmail, who must be programmed to accept mail for that domain.

Forwarding, on the other hand, made it possible to do a simple redirect of one address to another (eg. ~/.forward) but strict SPF rules will deny the forwarder as a valid source for the mails.