The copnnection of the transformer viz. Ddo or Dy5 does not have anything to do with the emission of gas . The gas emission is due to the decomposition of Oil or Insulation inside the transformer .
The phenomane of gas is primarily due to voltage stress and current stress in the transformer . Considering that the gas comes out even at No-load , there is a voltage stress inside the transformer . Pl check the insulations and the clearnces of dofferent high voltage points to know the source of the gas. This can be attended by improving the insulation .
If the gas comes out only at a partaicular load , then it is a current density or Magnetic heating prooblem , which has to be attended differently .