If not for the fact that he smashed in Chinjao(The drill head guy)'s head with 0 difficulty, I'd actually argue for the possibility of him being a scrub. There's a lot of factors that were already mentioned to be considered. Right now I feel it's most likely that he's Yonko level though. As someone who thought PK level didn't exist, this chapter is one that has changed my thought on Roger the most.
Garp had help taking down Rox, but this reveal gives more validity to his strength than doubt. You look at the fact that Oden, who was apparently far stronger than the commander's we've been seeing, was taken on WB's and Roger's ships and it becomes clear powerlevels were never lower. There may be a greater amount of powerful people, but it seems they still haven't hit the peak that those of the past set. For example, when Big Mom implicates inferiority to WB and Kaido implicates that WB was his last true challenge. Big Mom and Kaido can have only descended from whatever their prime was just as Rayleigh, Garp, WB, and Sengoku have.
Add the fact that Garp played cat and mouse with Roger(Prime WB's equal) AFTER Rox's defeat and there's a case to be made that Garp is around PK level with WB and Roger.