Depends on when it happened. And no, you don't have to be relative to do that. Zoro tried to kill Enel and vice-versa, and they weren't relative when it happened. I see where you're getting at, but the logic isn't necessarily valid. The most you can say about Garp is, he was strong enough to give Roger a challenging fight, which applies to about any given top-tier. It's possible Garp can push him more than others, like, say Sengoku, but he was never quite equal to Roger like WB was, period.