I have a floor pump with the pressure gauge. When I attach it to the the tube having Schrader valve, the gauge shows the pressure. But when I attach it to Dunlop valve, it shows zero at still stand.
Am I expected to see the correct pressure reading with Dunlop valve also or is this something common for this type of valve and I need some technique to estimate the pressure anyway?
You're not expected to see the pressure.
A Dunlop valve has no means to disable the check valve. You empty the tube by unscrewing the entire valve, not by disabling the check valve.
A Schrader or Presta on the other hand allows access to the check valve mechanism, so you empty the tube by disabling the check valve, and also the pump disables the check valve when you attach it to the valve.
The only way to know pressure on Dunlop is to pump until the pressure at the pump exceeds the pressure at the tube, in which case the valve automatically opens and you know the pressure. You know it because the rate of pressure rise is fast when compressing air only at the pump, and when the pump is finally connected to the tube via the check valve, the rate of pressure rise reduces markedly.
This problem of Dunlop valve is one reason not to use one. If you have a Dunlop valve, you can buy a Schrader tube and it will fit there just fine. Presta would optimally require two adapters, one at the tube side to prevent large hole from puncturing the tube, and one at the valve side to center the valve in the hole.
