For Saitama, if I tried to build him in GURPS*, I'd give him a Metatrait and make it into a Wildcard Power. The Metatrait would consist of a crushing Innate Attack with ridiculous damage and a large pile of DR and IT:DR (Cosmic: Round Down). As a Wildcard, it's name would naturally be "One Punch!" (ideally, stated like the first line of
the anime intro). Pretty much all of his demonstrated abilities - Tunneling, Ranged Innate Attacks that level mountains, Enhanced Time Sense + Altered Time Rate, etc - would simply be aspects of that.
One thing to keep in mind - turning something into a Wildcard Power has the same cost as giving the character 11 abilities, related and equal in points to the base, and setting them all as Alternate Abilities to each other.
*
I wouldn't, honestly, just as I wouldn't try to actually make Superman. The greatest, most admirable hero of One Punch Man might be interesting to stat up, however.
I speak, of course, of Mumen Rider.