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.