Is it possible to use GTM in front of Web Services without impacting users in a failover scenario?

We have active-active datacentres and the networks guys want to use GTM to be able to effectively "drain" a datacentre.  While I can see how this would work to a certain degree by following the guidelines on F5's website. I don't believe it would be possible to effectively fully failover to the second datacentre by the use of F5 technology alone.  My understanding is that CSM failover would still need to be manually invoked?  Is this the case?

If we were just to follow F5's documented approach, what would happen when a user who is normally homed on a Front End server on Site 1 was routed to Site 2 by GTM?

Is Site 2 an entirely different pool?  What's being failed over, just the web services or the entire pool?  I guess what I'm asking is, what's up and what's down in this scenario, is it a full hard failure of the site or just the web services and are the Lync users manually failed over in this scenario?

They want to do a full hard failure of the entire site