Hello @Litefilos nice to meet you.
Please check this topic Google Authentication issue: Status Code Error 400 - Questions & Answers - Make Community to understand what is happening.
After you have read the topic please use this link for documentation because from what I see the link from that topic is not working Connecting Integromat To Google Services Using a Custom OAuth Client Help Docs | Integromat Help Center
If you need additional support, please don’t hesitate to reach out.
//VLAD