When asking "Is gravity impermanent?", it may be worthwhile distinguishing between a couple of points, all of which may come under the broad idea of "gravity" in non-specialist terms, but which for the sake of this question, may lead to very different answers.
1. Gravitation - in the Newtonian sense. Newton's law of universal gravitation states that every object in this universe attracts every other object with a force which is directly proportional to the product of their masses and inversely proportional to the square of distance between their centres.
2. Gravitation - According to general relativity, the observed gravitational attraction between masses results from the warping of space and time by those masses.
3. The specific gravitational force exterted by planet earth upon objects, such as that which stops us flying into space.
Moreover, wrt to Buddhism, when Buddhism is making statements like "sabba-sankhara anicca", or "sabba-dhamma anatta", etc. all these sankhara and dhamma are referring to phenomena, and not to the laws relating phenomena. For instance, paticcasamuppada is not considered a dhamma in the sense of a phenomena (by most schools), even though it is considered a dhamma as an abstract principle. The principle itself is paticcasamuppada, whereas the objects subject to this principle are paticcasamuppanna. Quite different, and not to be confused.
So, whereas the specific force that, say, makes my body "fall downwards" is one thing, whereas the general principle that there is going to be an attractive force between my body and the earth, is another. The former will vary, with my own mass, the mass of the earth, and the distance between myself and the (center of) mass of the earth. However, the principle itself does not.
My recently moved Blog, containing some of my writings on the Buddha Dhamma, as well as a number of translations from classical Buddhist texts and modern authors, liturgy, etc.: .