Pros:
Makes him stronger in long, drawn out fights since it allows you to fully stack it
Gives him a better 1v1 potential and split-pushing threat, which is pretty much what the champion is all about
Increases your AOE(Area of Effect) in Teamfights or 2v1's
Gives you more waveclear (Same has increasing Split-pushing capability)
Cons:
Doesn't provide you any real stat( You have to Auto-Attack to gain the benefits)
Pretty much only increases what he is already good at(2v1 , split-push threat), at the cost of one item and doesn't cover any of his weaknesses( Heavy CC, being targeted or picked off)
Overall:
While it isn't the worst Item for him, personally i wouldn't do it unless i was really fed already, feel that a Sterak's or Maw would be better to provide you with more survivability in a TeamFight or in a 3v1 situation
Regarding your final Build :
I think you are going a bit too overkill on the Damage, you don't need Essence Reaver or BloodThirster, my personal favorite build for him is:
Blade of the Ruined King, Stattik Shiv, Infinity Edge(if ahead) or Ravenous Hydra( If im going for split push), a Sterak's, and then some Situational Items, if they have lots of CC (Crowd Control), i would go for Mercurial Scimitar, if they have Tanks i would go for a Lord Dominik's Regard, if they have Squish i would go for more damage, all Depends on the comp you are playing agaisnt really
If you really want to replace Guinsoo's onto your final build, just remove Essence Reaver, it doesn't benefit you has much has the Guinsoo, although i don't completely agree with your Overkill damage build, but hey, opinions