These are markdown wrappers that agent is adding, you can specifically ask it not to do this in message/thread, something like: “your entire response/output is going to consist of a single JSON object {}, DO NOT wrap it in JSON markdown markers”
You could also consider calling Gemini agent through API, it will not have the convenience of Make.com agents but might give you additional tool options to force output format (I know openAI assistants have this, I’m not sure for Gemini)
You can also remove markdown wrappers manually, also see previous post: