At the end of 2018, Microsoft officials said they were working on a Google G Suite to Office 365 "migration experience" that would be ready by Q2 2019. As of this week, those tools started rolling out, with full availability promised over "the coming weeks."
Microsoft's documentation page entitled "Perform a G Suite migration" notes that in order to perform a staged migration, users must have Office 365 Directory Synchronization (DirSync) set up or they will need to manually provision allof the MailUsers outside the migration process. There's an overview of the process using one of Microsoft's favorite fictitious companies, Fabrikam Inc. as part of the documentation.
In its Frequently Asked Questions (FAQ) post about the migration tools, the Exchange Migration Team answers questions about commonly used terms, the migration process and more. There's also an introduction to Exchange Mailbox Replication Service (MRS), which handles mailbox import, export, migration and restoration for Exchange and Office 365.
Admins will be able to tell if the new migration features are available in their tenant by seeing the G Suite (Gmail) migration option under the Migration tab in the Exchange Admin Center (EAC) or gaining access to the -Gmail parameter for New-MigrationBatch CMDlet, according to Microsoft's blog post.
Google already has an Exchange migration tool. And while there already is a way for admins to migrate IMAP mailboxes from G Suite using migration endpoints, this option doesn't migrate contacts or calendar items in an automated way.