Question: Recently, I read a book called "Swallowed Star". In the story, a virus changes human DNA enough for humans to have unbelievable superpowers and skills. Some even obtain the ability to have their own gravitational pull and not breathe in space.

I want to know if it is actually possible for genes to be able to cause such things, or is it just impossible theoretically speaking if there isn't enough matter?

PS. Sorry if this sounds like a stupid question. I don't have the knowledge of space and viruses to answer this myself. Please forgive me.

Answer: 
There is no conceivable way for a change in a human's genes to enable them to alter the laws of physics (i.e. give them their own gravitational pull).

Not needing to breathe for extended periods could be made possible with some form of extremely efficient [anaerobic respiration](https://en.wikipedia.org/wiki/Anaerobic_respiration), which provides a body with energy without requiring oxygen. Currently, there are no genes in humans which code for a choice between anaerobic or [aerobic respiration](https://en.wikipedia.org/wiki/Cellular_respiration); whether an organism has [metabolic pathways](https://en.wikipedia.org/wiki/Metabolic_pathway) for breathing with oxygen (aerobic) or without oxygen (anerobic) is much more complicated than an on/off switch.

However, [modern gene editing technology](https://en.wikipedia.org/wiki/CRISPR_gene_editing) can splice foreign DNA segments into preexisting DNA strands, and has produced things such as [goats whose milk contains useful spider silk](https://phys.org/news/2010-05-scientists-goats-spider-silk.html) or [strawberries with flounder antifreeze genes for cold resistance](https://www.nytimes.com/2000/12/05/health/personal-health-gene-altered-foods-a-case-against-panic.html). Sometimes, it's done via artificially-altered viruses. It is therefore clearly possible for a virus to splice foreign DNA into an organism's genetic code in a manner causes physical changes (as opposed to simply being [non-coding ["junk"] DNA](https://en.wikipedia.org/wiki/Non-coding_DNA) which doesn't appear to do anything).

Essentially: there's absolutely no way people with their own gravitational pull are possible with anything that follows humanity's current understanding of the basic physical laws of the universe, but, conversely, I'd say it's *quite* likely that, even in real life, gene-altering technology will eventually enable altered people to breathe for *long* periods (if not indefinitely) without oxygen, and our gene-altering technology today already uses viruses on a regular basis. Big no on the gravity, medium yes on the lack of a need to breathe.

Dealing with the dangers posed to the human body by the hard vacuum, cosmic rays, and extreme temperature differences found in space, on the other hand, might be somewhat more difficult. And don't forget that even a human whose bodily processes run off of super-efficient anaerobic respiration will need to breathe oxygen eventually...
I would argue that a sufficiently complex (that is, likely not natural) retro-virus could make some changes like super-strong, super-fast, likely significantly improved senses of sight and hearing. Even there it might only express (either fully or even partially) in children rather than already-grown individuals. And to effect children the retro-virus would need to infect gamete-producing tissues.

To do away with the need to breathe you would need to integrate an entirely different metabolic pathway, I suppose technically feasible, somehow, but seems unlikely. Even more unlikely for that alternative to be as energetically-favorable as using free oxygen from an atmosphere (that is, just as jet engines have better performance than rockets because they don't have to carry their oxidizer along with them). However, none of that seems as unlikely as modifying gravity.

None of those things are 'magical'. Basically imagine the most optimized organism you can think of along any given axis and it could probably be accomplished.