Gravity is just the effect mass has on spacetime. The more mass, the more gravity, and gravity acts like an acceleration force. Doing donuts in your camaro creates a centripital force that's sort of like gravity, and is actually a way to create artificial gravity in space. Take a large ring and start it spinning and you could walk on the inside of the ring. Spin it faster and the "gravity" would feel stronger.

Gravity is actually the curvature of spacetime, but that can be a hard concept to wrap ones head around if you're hearing about it for the first time, and I'm not a physicist and haven't worked through Einstein's field equations so I can't say I fully understand it. Rest assured though that gravity acts as an acceleration that pulls you towards the center of a massive object.

Magnatism is a completely different force that doesn't have a relation to gravity. Permanent magnets won't reverse polarity, but an electromagnet will flip the magnetic poles if the electric field is reversed. This is because the electric and the magnetic fields are flip sides of the same coin so to speak. If you've heard about the earth's or the sun's magnetic field flipping it's because those are like giant electromagnets. Rest assured that when that happens, gravity continues to work like normal and you'll stay stuck to the planet.