1. Game Ready is the name that Nvidia calls their drivers. What should concern you more is the version of driver that you are running. The current version as of this post is 397.93 (WHQL).
2. To put it very bluntly, a driver is what allows your hardware to work and communicate with the operating system. They are definitely worth it, if you want your card to work.
3. Yes, it can make slight (but negligible) impacts on gaming. But with a card that you have, it shouldn't bother you too much.