You click the link to access Sharegate Online (https://app.share-gate.com), and you see this screen:
Sharegate Online requires you to already be signed into your SharePoint Admin Center as a Global Administrator before you try to access it (by default the address should look like admin@[TenantName].onmicrosoft.com).
If you are signed in with any other credentials, you will see the above error.
For more on this, please see here.
Sign in as a Global Administrator and try to access Sharegate Online again.