Does a GitBook page need a gitbook.io address to be genuine?
No. GitBook supports custom domains, so genuine documentation does not have to use a gitbook.io address. The reverse also matters: a GitBook hostname or familiar page design does not establish that the publisher represents the project named on the page.
The custom-domain documentation explains that published sites have a default GitBook address and can use a chosen domain instead. A hostname difference can be a normal publishing choice, not proof that either page is an impersonation.
Find the documentation link supplied by the project’s established website or repository and compare its complete destination. The GitBook profile guide concerns public references; it cannot certify the authority of instructions on a documentation page.
If a copied guide changes an installation or login destination, record that actual difference along with the source reference. Keep the publisher association and the safety of any instruction separate. Recognizing the hosting platform answers neither question by itself.
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