You lose both ways. The council can say the property counts as yours, so qualifies as an asset, whether or not you make a gift of the rental value to your grandmother. Inheritance tax will still be levied when the lady dies, since the gift is a "gift with reservation" - unless the present inhabitant pays a market rent. According to inheritance tax rules, a "gift with reservation" is not a true gift, and full inheritance tax will be charged. I know this because my father tried to do exactly the same thing, even though I warned him it wouldn't work. And it didn't.
In fact, you may lose three times, since the council can claim ( if she has to go into care) that the lady gave you the property in order to avoid nursing home bills. If so, they can still count the house as her asset and claim back the fees ( or some of them, anyway)