Did a search for exhaust smoke (both gray and white) and found the posts re this subject to be lacking. Hopefully, we can get a good list of reasons and cures for different types of smoke.

Blue smoke: oil...

White smoke: lean, water vapor...

Black smoke: rich...

Gray smoke: bad fuel injector, bad gas, bad O2 sensor...

I could be wrong on some of the reasons (hopefully you guys will help correct me). For my car, I had gas that has been sitting in the car for 2.5 yrs (car was down for repairs) and went bad. Emptied the tank, put some fresh gas in it, added some fuel injector cleaner, and it cleared up almost immediately.