He wasn't lying about being willing to spend 100 years researching the gateway. He had the time and the means to spend that much time on a single task.
Even if he didn't have the time, he would make the time for something this important. Gaining access to the league of world eaters means that he might not need the help of the people who cultivated the Everlasting Wood Art to help him replicate the famous living metal material by using the silver pine trees to derive it.
If he can join the league of world eaters, he might even be able to get living metal right from its source. So this matter if very important to him and worth sacrificing 100 years to achieve. Fortunately, he has that much time to spare.
As a nascent soul cultivator, he has a normal lifespan of 1,000 years. This is just the average lifespan, though. There are many ways to improve lifespan, even by 10%. So he has a lot of time on his hands.
