Gmail uses a .com domain extension. Gmail is the free email service offered by Google. The full domain name is "mail.google.com" and it is free.
All Google services use a .com extension, including play.google.com, accounts.google.com, and maps.google.com. Gmail is perhaps the most popular Google service; as of 2014, it accounts for more than 49 percent of all traffic to the main Google domain.
For search engine optimization (SEO) purposes, search engines generally do not care whether an extension is .com, .net or .org, provided the site's on-page search-engine optimization and other factors that influence page rank are solid. There is no real advantage that any of the extensions has over the rest in terms of SEO.