Well basically any object having mass bends the space time fabric but its very negligible to be noticeable. The higher the mass of an object the, more it bends this fabric. Now the mass of a body in motion is calculated by the following formula..
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where m relativistic is the final mass of the object in motion, m rest is the mass of same object when at rest, v is speed of object, c is speed of light.
Now if you replace the v with a c, that is the velocity of the object in motion reaches light speed the m relativistic becomes infinite bending the space time fabric noticeably. So you're not wrong to say that an objects mass causes the space time bend but any object with mass is able to bend the space time noticeably if its moving at a relativistic speed.
So object with mass moving at relativistic speed does bend the space time to be noticeable. Hope this clears it up for you. And again the laws of physics in NV are similar but not always the same but unless stated or shown to be different they're the same. So based on that Night Guy bending the space only indicates that Gai was moving at relativistic speeds. Ofc, for the readers entertainment and understanding Kishi made it as a flashy dragon and not something that happened in a instant while showing the effects of the attack at the same time.