This is a top lane role, do not bring this jungle, this build can be played mid as well. The goal of this build is to poke the enemy laner and to be able to act as a second jungler (with the smite).

The gunblade and GLP work well together because they are both hextech items which mean they have a hidden magic pen passive which rek sai needs for her damage on her q. The secret passive on hextech items was found by a Korean challenger player who climbed to rank 1 this season and through detailed coding found the secret passive which allowed him to steadily climb to the top of the ladder.

Comet is key for the main portion of your damage with your poking Q
Nimbus cloak makes you fast and works well with celerity
Celerity is OP rn
WAterwalking is also OP
All runes work well with celerity if you didnt notice
Magical footwear becasuse it saves you money
Cosmic insight because you need to be able to shoot Q's faster

This build is nearly unstoppable in the right hands so play it well
A video of this god build may be published soon so keep an eye out