According to our current understanding of physics, time travel, particularly to the past, remains a topic of speculation and is not yet known to be possible. While it is true that gravity can affect the flow of time, the concept of using gravity to manipulate time for the purpose of time travel is still purely theoretical and has not been demonstrated in practice.
The phenomenon you mentioned, where strong gravity can slow down time and weaker gravity can speed it up, is a consequence of Einstein's theory of general relativity. According to general relativity, gravity is the curvature of spacetime caused by massive objects. In the presence of strong gravitational fields, such as near a black hole, time can be dilated or slowed down relative to a distant observer. This effect is known as gravitational time dilation.
However, it's important to note that gravitational time dilation is a relative effect. It means that an observer near a massive object would experience time differently compared to an observer far away from it. For example, if someone were to venture close to a black hole, time would appear to slow down for them compared to someone observing from a safe distance. But this does not allow for traveling to the past.
There are various proposals and theories, such as wormholes or the concept of "closed timelike curves," that have been explored in science fiction and theoretical physics as potential mechanisms for time travel. However, these ideas often involve speculative or hypothetical scenarios that require exotic forms of matter or energy that have not been observed or confirmed in reality.
It's important to approach the topic of time travel with scientific skepticism. While our understanding of the universe continues to evolve, and future discoveries may bring new insights, at present, time travel remains firmly in the realm of speculation and theoretical exploration.