E90 320i N46 Expansion bottle

Tell me this lads should one be able to remove the cap off the expansion bottle when car is idling after it has been driven for a 20 minutes?
Obviously you'd have to screw it off slowly to release the pressure but in doing so if it starts gurgling out water whilst doing so I assume there's to much pressure and could be a symptom of head gasket failure?

u can't open after 20 minutes of drive, its under full pressure .u must wait ,let it cool down , and only after ,can open it ,and slowly .not common for head gaskets .once its not overheating to should be all ok