Magnes has the ability to manipulate gravitational fields through the manipulation of his own gravitational field. He has a roughly 30 foot default "orbit", and things within that orbit can have their gravity lowered, raised, shifted back, forward, up, or down, or spun around his orbit. The direction that he can move things in is very stiff, and there's generally no fine control of moving things around due to the fact that it's gravity rather than telekinesis. So while he could rip a door handle off, he can't really turn it in an easy way without breaking it.
When he changes something's gravity and it leaves his orbit, it generally keeps moving with those changes. So if he shifts something to fall up, it'll keep falling up for a very long time before it falls back down. In extreme cases, he has the capability to make permanent gravity changes, but to this day he has no idea how to actually do it on purpose or how it actually works in order to practice it, so usually these changes happen by accident, and he's never done it on a large scale, only to small objects accidentally. He also lacks the knowledge to shift this effect back, as that in of itself is also a permanent change.
In combat, he's commonly seen increasing his gravity to crush things, make it difficult for people to move, and other such uses of his ability. The way that he makes his own gravitational field heavy, is by making his own body heavy. The core of his gravity is located at his abdomen. From here his entire body increases in mass dramatically, though he himself doesn't perceive the change in mass, he moves like normal, his body is perfectly adjusted, but the world around him sort of reacts like he weights multiple tons. This in turn makes his gravitational field crushingly heavy.
The opposite is also true for his gravity, for his field to become lighter, he also becomes lighter.
He uses his gravity for numerous movement capabilities, such as walking on walls and jumping very high. It's important to remember that his flight isn't actually flight, it's him shifting his gravity in a different direction, so that he falls in the direction that he wants to "fly". He can only move at the speed of falling, which is significantly slower than someone with real powered flight, and has significantly less avenues of control or fine movement.
His senses have also been important since the time he became a black hole. He can generally sense what he calls "gravity feel". The gravitational forces of various things, big and small. It can be overwhelming and most things are white noise. The more things are touching, the more they blend in with each other or get overshadowed by larger things.
He has a perpetual sense of where the Sun and Moon are in the sky, due to them being the largest forces tugging against Earth. Other feelings in the sky tend to be white noise, since he doesn't really understand them in context. So for example, he might feel Jupiter tugging against Earth, but he doesn't really understand it in context, so it's kind of ignored as irrelevant white noise. He can also sense gravitational "weaknesses" in space, though he doesn't yet understand what that means. He can sense areas where the link between two universes are touching closer than normal, these areas tend to have weaker gravitational forces. To him, they'd seem very odd to be near, but he wouldn't understand why they exist or what they mean.
As far as specific weights and things like that, Magnes' own body can become multiple tons heavier than a normal human body, which raises his gravitational field's weight to a disproportionately large degree. It's a bit like there's something heavier than Magnes creating the field, even though there isn't. It wouldn't make much scientific sense, but it most likely has something to do with string theory.
He can manipulate most things that he can fit into his 30 foot radius, it's mostly a matter of removing gravity for larger things. But moving something larger than that 30 foot radius takes a ridiculous amount of strain. Like, stopping a 50 foot tractor trailer with his gravity field would cause a legitimate strain, because there's a bunch of other weight that he can't manipulate, and it's just… there's a lot of tricky physics involved.
A major flaw of Magnes' is that his ability functions on a distinct lack of doubt. The more he doubts his ability, the less effective it is, the more he believes in his ability, the more effective it is. Notable is the fact that early in his history, he had a severe lack of confidence in himself and his ability was largely touch based. People like telepaths and people with emotional manipulation could effectively render his ability useless.
And since the overall emotional trigger for his ability is the belief that he can do something, it means that if he were, say, falling from a huge height, and thought he was going to die, he would smash into the ground, because the lack of faith in his ability would render it non-functional.
Magnes floats when he's unconscious, making him similar to an astronaut in space. He's significantly lighter and easier to move around while unconscious.
