I would ask "why is it bad" or "what evidence supports this claim?" How has it been driving/acting? Have you ever had the transmission serviced? If not, maybe a fluid and filter change may make a difference before jumping on the repair wagon.
On the other hand, if its slipping, jerking, shuddering, or howling, then they are perhaps on the correct path.
If it needs to be replaced, then I would see what kind of GM loyalty, customer care, they can provide. A transmission done at 76k and no warranty is complete BS in my book. I would see if they will goodwill all, or a portion, of it. If that's not an option, then ask what a new transmission price would be (try the goodwill cost route) instead if you plan on keeping the vehicle for many more years. A GM replacement transmission will carry a 3 yr/100k nationwide warranty. So, depending on the cost difference, you should weigh your options.
If it's even remotely close in price, I'd go new transmission. Just make sure its documented as "customer pay" not warranty so you get the full warranty on the transmission if you are footing most of the bill. Get it in writing beforehand what the warranty will be. They may offer to pay some of the bill but then stick you with a 2 yr/24k warranty that could leave you in the same spot.........again