A lot of people want Luffy to go Gear 5 to have a chance against the top tiers. However I don’t think he really needs any new gears to fight against them.
Oda is focusing on improving Luffy’s base form. This is confirmed by Luffy’s recent fight against the 1st Yonko Commander, Charlotte Katakuri, wherein Luffy, for 97% of the fight, was only using his base form against him. Because of this his base form and additionally his Observation Haki have improved to new levels.
Katakuri couldn’t do anything against Gear 4 Bounceman. I know some people will disagree with me but Bounceman was stronger than Katakuri in every way except for Observation Haki. His Observation Haki was what saved him from Bounceman. If Gear 4 is this much stronger than 1st Yonko Commander even when it’s nerfed than it clearly should be put in a more fitting rank. Also don’t forget Doflamingo which was completely fodderised by Gear 4. Do you know anyone else in the OP Universe that could do that? Of course it’s the Admirals and the Yonkos. This is why I think that Gear 4 on its own is Admiral level whether that be low tier admiral level, doesn’t matter.
I think for now all Luffy really needs to be able to defeat the top tiers is master all 3 Haki, Gear 4 and Awakening. Some might say that Awakening is going to be Gear 5 which I don’t really mind if that’s the case. Anyway, those 3 things will put him on a seriously another level. No gear 5, 6 or 7. Oda seems to think the same too by giving Gear 4 its own forms like Snakeman and Tankman.
*Theory by Atheistsomalipirate