I think there are really three different things in play here.
First, if you're just planning to dump the grain into a bucket, yes you want food grade. If you can get the buckets for free from a bakery or Costco, great, but there probably is a bit more risk they aren't entirely suitable because you don't kow how the employees used them.
Second, storing grains in buckets alone is probably fine, lots of people here do it. But adding a mylar bag to the bucket will greatly increase the storage life of the grains, if you do the mylar storage thing correctly. Which is beyond the scope of this thread. If you use a mylar bag, it doesn't matter if the bucket is food grade or not. I actually have mylar bags filled with food and stacked on shelves without buckets, but I don't have any buckets without mylar bags.
Third, the gamma lids are a lot easier to open and re-close than the regular lids. But they're also a LOT more expensive. I don't really see the point of the gamma lids if you're just putting away some grains for a possible future emergency. On the other hand, if you just want to (hopefully) keep the mice out of the stash of rice you regularly eat, it would be a good idea to get a gamma lid for that bucket.