require("CCS811").connectI2C(i2c);

Oh, there is a difference. If you type gas.get() in the REPL, it prints out the result of the last statement. But if you call it somewhere in your code, it does not.
So it should be print(gas.get()); in the interval:

var i2c = new I2C();
i2c.setup({ scl : D5, sda: D4 });
var gas = require("CCS811").connectI2C(i2c);
setInterval(function() {
  //gas.get(print);
  print(gas.get());
  //gas.on('data', print);
}, 3000);