Bakman2 not in the gauge node this is not mustache
As your incoming payload is a buffer which means you probably have the output of the serial node set to buffer not string.
To output a JSON string from the Arduino code try.
Serial.print("{\"Temperature\": ");
Serial.print(temperature );
Serial.print(",\"Humidity\": ");
Serial.print(humidity);
Serial.println("}");
Which should create a JSON string
Then make sure the serial node is outputting a UTF8 ascii string not a buffer
Then pass that through a JSON node and then you can enter {{msg.payload.Temperature}} in the value format field.