whistler,
The gray buckets are not food grade unless they are labeled as such. Lowes does sell white food grade plastic pails made by Encore who also makes the gray buckets. Other colors can be food grade, but typically they are white and need to be labeled as such.
If it isn't clearly labeled marketed and sold that way, it isn't.
Keep in mind that the food grade label is not the "end all be all" and food grade plastics are not completely safe. Nor are all white buckets food grade.
Case in point, BPA was in all polycarbonate up until a few years ago (which was/is food grade according to the FDA) .
All food grade gets you is the knowledge that you have brand new, un-recycled material.
Will there be leaching? probably. depends on the type of plastic. Same as any other bucket.
Does any of this matter with respect to growing tomatoes as opposed to storing food in them? I have no idea.
I don't want to discourage you from using the HD buckets, many people grow in them and have been doing so for years! Informed choice is all this is......
Here is a good link, aimed at brining but it does spell out a great deal of info on plastics and food grade.
http://www.virtualweberbullet.com/pl...html#foodgrade