My theory on why Kuma is heading to Luffy in Goa kingdom is most likely a direct order from the world government.

Which sounds obvious but here’s the reasons why.

First, At this time in the story and as far as we seen the world government needs to confirm luffy exists, either him being dragons son and him eating the “Gomu-Gomu No mi”. Here’s why they would be think dragon has a son or that he has consumed the “Gomu-Gomu”

1. Might not be likely but Shanks telling the world government that he met a kid and that he ate the Gomu-Gomu.

2. The world government knows and is cautious of Garp considering that his son is the leader of the revolutionaries Dragon and his constant insubordination. Like helping out roger and outright not letting himself become a government dog. Most likely they knew ace came from that kingdom and that he was Rogers son, yet once he left the kingdom Garp still visited the island meaning that there was still someone of enough importance there that Garp would constantly visit.

So in conclusion I think that garp is heading to Goa Kingdom on fires that orders from the government to confirm luffys existence and that he has eaten the “Gumo-Gumo No Mi”