That's very true. Do NOT do it as a separate landing page or you will ruin your google traffic.
The trick is to put your cookie policy AND your warning in a hidden DIV at the bottom of the page and use Javascript to display it over your content. That way it doesn't affect what google sees.
Also, use "Implied Consent". In other words, as soon as you display the message, set the cookie that says they have accepted. Don't make them tick a box or click a button etc. That way, if they do anything other than close the browser they have accepted your terms. Also it means you are not constantly hitting them with a popup until they explicitly accept your terms
You can see this in action at https://www.tacamateurs.com (note if you've previously visited the site then the cookie will already be set and you wont see the warning)
By the way, this wont satisfy Age Verification laws. That is something completely different

