Youhu seriously pondered the matter for a long while, wondering why Gamamaru believed he was the Son of Destruction.
The first possibility was that Gamamaru had never seen Youhu in his prophecy and believed he would interfere with the future's development. Thus, he had simply assumed Youhu was the Son of Destruction and wanted Jiraiya to eliminate him.
The second possibility was that Gamamaru knew about the reincarnations of Asura and Indra and had mistaken Youhu for the Indra Reincarnation.
After all, the true Child of Prophecy, Naruto, was the reincarnation of Asura. It was perfectly reasonable for the Indra Reincarnation to be the Son of Destruction. The two brothers had been fighting for over a thousand years, after all.
Every time Indra tried to cause trouble, Asura would lead his companions and defeat him while shouting slogans about "justice" and "friendship."
The time Hashirama Senju reincarnated was an exception. Even if Uchiha Madara shouted slogans about justice and friendship, he still had no way to defeat the monster that was Hashirama Senju. He was simply an overpowered stat monster.
The third possibility, and the least likely one, was that Gamamaru suspected Youhu was an Otsutsuki.
Gamamaru merely possessed the ability to prophesy, yet he had consequently developed senile dementia. He was completely different from the other Great Sages, who were all in the prime of their lives.
Meanwhile, the Otsutsuki only had to pay a tiny price to use Treasured Tools to travel through time and space and alter history.
In other words, Gamamaru most likely couldn't see any future involving the Otsutsuki.
That was why Youhu's appearance had frightened Gamamaru so much.
After considering everything, Youhu felt the second possibility was more likely. Gamamaru believed he was the Indra Reincarnation, but he had never expected to one day