Yes, see this article:
http://www.marketingrockstarguides.com/dkim-spf-and-cname-setup-for-marketo-289/
and
http://community.marketo.com/MarketoArticle?id=kA050000000KyrDCAS
Generally what you do is alter the DNS records, which may or may not be on your GApps management tool.
Remember to tell Marketo about your SPF and DKIM setup.