Mailgun and Sendgrid are both awesome for smtp. Mailchimp is great for marketing campaigns. Mailgun/Sendgrid are great for sending from your own servers with your own templates. Sendgrid/Mailchimp are great for creating/managing/storing templates of emails and send them on a schedule, manually etc
Neither one of them is a direct replacement to gmail where it brings another type of value (provisioning accounts, web client, docs/sheets/drive integration, 3rd party login [oauth]).
Microsoft has a more mature enterprise solution but it's more complex than Google's offering.
We use Amazon SES, not as complete and seamless as Sendgrid, but it has all the basics, very well integrated if you already have everything on AWS and cheap.