I mean, gear 2 isn't strong enough to badly hurt Lucci, and gear 3 is too slow. This seems like a way to get past that.
Of course, Lucci's legs are weakened right now, so maybe there's no need for that. I think it will involve gear 3 and Luffy being giant though, as a send off to Saul or something. MAybe he can use Gear 3 and gear 2 to become a full giant for a second.
I think Gear 2 is strong enough to badly hurt Lucci in his current conditions. Before, he was able to defend and run if necessary. Now, he cannot run away anymore, so all he can do is to defend and against Luffy's consecutive very powerful strikes I don't think his defend will help him a lot.