How do I link an apple watch 4 to a iPhone 12 but the watch is still connected to someone else's Gmail account
How do I link a apple watch to my iPhone 12 with out the email and password
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You can't;
The person that you bought/got this watch from needs to follow all the steps on this page;
Remove the Activation Lock so that the buyer can use it.
If they can't or won't then get your money back or recycle the watch and mark it down as a bad experience.