In my understanding—Being someone's Gilsangcheon is basically means that they're the source of someone's luck. Taeui being jaeui's Gilsangcheon is vital for his life. It was revealed in the recent chapters that whenever Taeui got sick, Jaeui suffers the same fate— Jaeui suffers worse health issues because Jaeui's frail real body relies on Taeui's strong life force and luck. That's why if Taeui dies, he will too. It's like a twin bond/connection
I guess if he wasn't jaeui's Gilsangcheon, then Taeui wouldn't be needed in the UNHRO or smth (Jaeui being a genius in weaponry and all)
I get the luck part while I dont get it is how its related to plot specifically. If you remove this "I bring luck for my brother" the plot wont change at all. Its still will be about brothers bond and people trying to use Tae to find Jae. I dont get how everyone knows about it, constantly keep reminding readers about it and just nothing related to Gilsangcheon part happens.

I still dont understand how Gilsangcheon part that ALL characters talk about it related to the plot.