Can i put a quarter inch smaller spark plug in my 2008 pontiac G6

Most pontiac have a gap pt .060 idk but u better stick 2 the right-hand sizes.

If you are talking about length, that can be a non factor. The real issue is the length from where the plug meets the engine block to the tip. The overall length of the plug can be unimportant. I have seen overall length changes in plugs from the same manufacturer and having identical part numbers. But the length from seat base to tip remained the same. That is what you need to ensure. Hope this helps. Also, make certain that the heat range is the same. Too cool or too hot can cause problems. Heat range is normally the numeric component of the plug.