Problem to display serial in data to gauge

Dashboard
11

Yes. Something like...

    char s[100];
    sprintf(s, "{\"humidity\": %f, \"temperature\": %f}", hum, temp);
    Serial.print(s);

↑ untested - but should get you on the right track

... then feed the payload into a JSON node & you will have a JS object

then, the docs will explain how to use the debug panel to find the right path to any data item.

https://nodered.org/docs/user-guide/messages