Once you get over $1000, though, one year warranties are a lot less common. The majority of $1000+ home projectors have at least two years warranty
Optoma provides a 3 month warranty on the lamp, which is the most common lamp warranty. Lamp life claimed is 3000 hours at full power, and 5000 at low power, so 500 hours while pretty standard, isn't exactly a whole lot of piece of mind, regarding an early lamp failure.
This is your call: While I'm not a huge fan of 3rd party warranties, I have to recommend buying a 2nd year.
A major failure, say, after 13 or 16 months, is going to ruin your day, and may even turn a projector into a doorstop, due to the cost of repair. With two years total warranty, at least if it fails in year three, by then there should be a comparable projector for less, or a better one for the same price. And by then, it might be time for a lamp for your projector.
Bottom Line: The one year Optoma GT750 warranty is very average for this class of projector. Nothing more, nothing less..