A GTO does not need a Monte Carlo for a business case, it just needs other zeta/rwd sisters for GM to pull it off. The GTO's problem is the Camaro and the Camaro's problem would be the GTO.
Here is the issue. When developing the Camaro business case, the product mix is a significant factor. This refers to the premium GM will charge for the LSx option over the V6 or other high performace variant.
That profit will factor into the whole program and balance the questionable profit at the lower end of the product mix.
So now, if GM decides on a GTO for Pontiac, those sales begin to eat into the profitable sales range of the Camaro, especially at the highly profitable price point. Those 10-15k highly profitable sales lost to a potential GTO on a less profitable vehicle could very well make or break the Camaro. Any other additional coupe variant, if it be Chevelle or Monte Carlo will only eat into the Camaro's sales.
So any additional coupes off zeta that is priced along side the Camaro ANY WHERE near the price point threatens the profitablity of that program. And if GM decides to develope a GTO and push the price point higher, than the Pontiac begins to encroach on Cadillac and their coupe.
A GTO is possible off of zeta, the key factor will be to cut development (tooling) to offset any lost Camaro sales and most likely that means, a GTO would be a limited volume vehicle and a higher price point than the current vehicle or GM will change the formula completely so that it does not compete directly with the Camaro on concept at all. With that statement I refer more to the Dodge Charger type of approach where there will be a G8 and a GTO will be just a model of that body style. But in that case, it would not be a coupe as we currently know the GTO.