Anyway, the gap between Luffy and Zoro is big!
Well, sure it is. Zoros end goal is to best one of the current top 3 (perhaps top 5) bounties in the world. Luffys goal is to be the best of all time. That's a gap right there.
If bounties are anything to go by you have
5.1B+ Dragon (presumed higher than WB)
4.04B Shanks
4B Akainu (presumed higher than admirals)
3.99 Blackbeard
3.59 Mihawk
Zoro will be the WSS (and perhaps the best swordsman of all time). But like Mihawk said PK > WSS. And Luffy is not just aiming for PK, he is aiming for GOAT even above Whitebeard, (presumed Dragon), Roger and Joy Boy.
You know, back in 2009 Tyson Gay broke the American record and ran the third fastest 100 m race of all time in 9.71. Problem was that Usain Bolt ran 9.58, a HUGE gap. Just so much a super world class athlete can do against the GOAT.