If msg.payload has 30 in it you can feed that straight into the gauge. If that doesn't work then feed it into a debug node and show us what you have. Also worth reading the node red docs page on Working With Messages.
However, if it turns out that actually msg.payload contains an array with one element, which contains the value 30 then you can tell the gauge to display the first element of the array in msg.payload by putting {{msg.payload[0]}} in the Value Format field.