Really Gamespot? 8.0? It's at least a 9.
The game starts off already with a bang, Marston with his family that he loves so much and discussing what ever happened to his uncle. Something is seen lurking outside the house, it strikes and bites Marston's wife Abigail, with no choice he kills Uncle. Abigail does not feel well, and eventually their son comes out to see what's going on, only to be bitten as well by Abigail. The zombie plague takes over both of them, and Marston goes into town to see what is wrong. There is an extension to the already existent map. As soon as Marston reaches the next town, it is evident... all hell has broken loose.
Same interface as RDR. Except ammo is valuable and is rare, as you set off rescuing survivors and saving/clearing towns from the zombie invasions you are rewarded with ammo. A headshot to the head kills it instantly, but you will notice in the first town that the zombies just keep on coming. Ammo can be found from looting zombies. The story of this DLC is brilliant, it is a great zombie themed story that crosses over with the western theme, and works very well. I haven't played much of the story yet, but as of right now it is pretty good, I'm at least 1/3 of the way through it, and I highly recommend it. It is only 10 dollars on the PSN, but if you don't have any of the other DLCs, I recommend getting the bundle for 20 dollars that comes with all RDR DLC. There is more to do, including hunting the mythical Chupacabra and Bigfoot. What was once Gang Hideouts, are now Graveyards, places that needs to be cleared from the hordes of zombies.
It even adds another game mode online, called "Undead Overrun". This is what I have been spending most of my Undead Nightmare time on. It is almost like Call of Duty's Nazi Zombies, but it has a different play style. You'll see exactly what I mean. Instead of Call of Duty Zombie mode which only gives 1 or 2 maps, RDR Undead Nightmare gives you 8 maps to play on with up to a team of four people. Yes, you do rank up and level up online by doing the Undead Overrun mode, you gain XP (more XP depending on your performance, decided by Gold, Silver, or Bronze).
Also, it adds another multiplayer game mode, "Land Grab". Land Grab is like King of the Hill, a little different. There is a post/territory that one must capture and defend for as long as they can. Once they are killed, or someone tags the post, that person now owns the territory. And so on, and so forth.
For 10 dollars, if you liked/loved Red Dead Redemption, this is a definite buy and is totally worth it. I highly recommend this.