In the fight between Geryuganshoop and Saitama, Geryuganshoop clearly states that not even Saitama could handle the gravitational pull of a black hole while he was using his telekinetic power manipulating the gravitational field.
Does this mean that he was creating gravitational forces equal to that of a black hole on Saitama? (which literally had no effect)
Does this mean that he was creating gravitational forces equal to that of a black hole on Saitama? According to an interview with the artist, Murata:
hm~, I'm not sure. Black holes seem too powerful. I won't know for sure without asking ONE.
Take note that his manipulation of gravity only happened in the anime as this was not depicted in the manga. In the manga, specifically in Chapter 34, their fight was swift (he was only able to hurl rocks/pebbles two times before Saitama threw a stone at his head, defeating him). Also, there is the possibility that Geryuganshoop either lied or miscalculated his own strength. I say this because he claimed to be the strongest telekinetic yet was easily defeated by Saitama throwing a pebble at him. He claimed to manipulate gravity and mimic that of a black hole but as seen, Saitama was unaffected. As @kaine mentioned, if that was really a black hole, Earth would've been destroyed.
As a side note, Murata also added that
...his ability to control flying object should be above Tatsumaki, because he can eliminate the friction between rocks and air. If Tatsumaki throws rocks like he does, her output will be too powerful, the temperature increase due to friction and pressure, and evaporate the rocks in an instant. Geryuganshoop's psychokinesis can eliminate the friction between objects and air, the rocks will fly at sub-light speed, that's the setting I based my drawing on.
So this means that Saitama can also withstand rocks thrown at sub-light speed, aside from the gravitational force by Geryuganshoop that is supposedly as strong as a black hole's.
We don't really know how strong Geryuganshoop's force on Saitama was. By Saitama's standards it wasn't really that strong.
Geryuganshoop has telekinesis which he used to try to push Saitama down to the ground. Saitama just remained standing and moving as if nothing happened. The minions fell to the ground. If the force was as dramatic as he would like to indicate, the result would be closer to what you would see in a hydraulic press. Little of that occurs. One of the problems is that such force is diffuse (a lot of force over a larger area just results in a small amount of pressure). This explains why he mostly uses it to throw projectiles.
The gravitational pull of a black hole is kind of vague as a measure of strength. I'm experiencing the pull of many of them right now. Even if you indicated that it was the force at the event horizon it is not necessarily that destructive. What ever he meant by that term, the forces we saw did not match the forces you imagine when you imagine a black hole's power. If, for example, he actually created a dime sized black hole, the entire earth would have been destroyed quickly.
Geryuganshoop's demonstrated power certainly more impressive than most. But it was still trivial compared to Saitama and other's he has battled.
That being said, if the Mangaka thought it would be funny, Saitama could get punched into a black hole like he was punched into the moon. He would land on... something... and then jump back to the fight. But the fight would be over by a millennium due to relativistic effects.
the fact is that it is claimed that it was the gravitational force of a black hole and that is irrefutable. why didn't it absorb the planet or anything else? simply because it created a gravitational field that only applied in that room.
Now that it is clear that the gravity was that of a black hole, in an average black hole a 70kg person weighs 100,000,000,000,000,000kg, so Saitama had that force (and more) so that gravity would not affect him at all. To that we must add the force with which he threw the stone so that it was much faster than light. the force required for that is infinite. Given this, I can conclude that Saitama's power is simply infinite, he is a satirical character who will have the infinite power necessary to do whatever the plot requires, even if this goes against logic and physics.
