Both "tension/struggle" and "gravity/force" are given in the dictionary tips above, but I found only "tension, struggle" in the translation of mvutano at http://www.elimuyetu.co.tz/subjects/arts/swa-eng/m.html
Since "tension" means stretched tight (or a strained situation) and "gravity" means the force that attracts a body towards the centre of the earth, I was surprised that mvutano could mean both. I suppose you could say that tension pulls in two directions and gravity pulls in one.
Or it is just that words sometimes have two opposite meanings, e.g. "screen" can mean to show (a movie) or to hide (an unsightly view).
Gravity is not only a force that pulls things towards the centre of the Earth. Every object with mass pulls on every other object with mass. The sun pulls on the Earth and the Earth pulls on the Sun. The moon pulls on the Earth and the Earth pulls on the moon. Even our own bodies pull very weakly on everything around them. The force of the pull depends on the mass and the distance between the two objects.
Tension and gravity are two very different forces as defined by physics, but there is an obvious similarity between them: they're both a pulling together of things, so seems perfectly fine and logical to me for them to be the same word for everyday usage ... as long as there is a way to distinguish them when you need to be really specific.