Migrating from Gmail has a few extra steps than with other email services. If you are planning on a migration from G Suite to Microsoft 365 or Exchange, then there a few additional steps to create API scopes that will be required to ensure all data is migrated, instead of just mail data.
Please see this guide on planning a G Suite migration, if you are looking to migrate from a gmail.com account, and only need to transfer email data, then you can read the following abbreviated guide to expedite your migration.
1. Log in to gmail.com and click the cogwheel icon at the top right of the screen.
2. Select See all settings in the menu that opens up.
3. Make sure that IMAP access is enabled.
4. Allow less secure apps. Make sure this setting is turned on. You can turn this off once the migration has completed. Gmail and Gsuite both have a bad time trusting our email migration software even when adding as a trusted app.
5. You can now proceed by filling out the email migration request form. You will likely still receive a Google notification that a suspicious sign-in attempt was blocked. You can click to allow this as this is the migration server trying to make a connection.