How can the sages be sure of what they’re doing with gematria?

I’m recently on the process of doing tshuva, becoming more orthodox in the religion and studying more about judaism. Of course, a lot of questions come up with that. I know for a fact that the gematria is a well estabilished practice in most orthodox sects, but how can the rabbis be so sure of what they’re doing? How do they know they’re not just making crazy maths of nothing? There are so many things outside the Written Torah itself in Judaism, that I thought there’s a possibility that it’s wrong. I mean, they might be sages, but they’re still human and humans can and probably will make a few mistakes. If there is no Kabbalah nor mysticism in the written Torah, then how did they come to their conclusions and studies? How do they know for sure that they’re doing actual meaningful studies with actual basis?