So Cosmic Garou is described as having gained knowledge of the "flow of all energy" and "behavior of all forces in the universe".
And thus he can copy any of those forces which he demonstrates by...
Copying Nuclear Fission with a punch:
The explosion/s generated from the technique are registered by a nearby ship as an actual nuclear attack...
And to top it off the end results of the explosions are mushroom clouds that are typical of nuclear explosion
He then copies Saitama's consecutive normal punches and seemingly clashes with the real deal evenly:
And lands a punch on Saitama at the same time Saitama lands a punch on him
And finally...
He copies a Gamma Ray Burst
And we get a detailed explanation of what a Gamma Ray Burst as the attack hits Saitama