Yes it can be difficult to to PSA on gypsiferous  soils as they tend to flocc out when using standard mechanical dispersion techniques. In the past after pre-treatment with acid and peroxide to remove carbonate and organic matter,  I have used careful washing (shaking and suspension followed by flocculation and decanting until gypsum is removed and the fine particles remain in suspension) (This is similar to the Loveday method in the FAO soils bulletin 62 - management of gypsiferous soils) . In terms of agronomic uses, whether or not PSA is necessary all depends on what you need to know. Some of the information that PSA may help you estimate (such as CEC, water holding capacity, hydraulic conductivity etc) can all be measured directly if necessary. If you need it for taxonomic purposes (USDA soil taxonomy for example), you may have little choice but to persist with trying to adapt methods to remove soluble gypsum prior to full dispersion. The FAO soils bulletin 62 has a number of different techniques that may be suitable. 

Thank you Dr Andrew for your answer and interesting clarification.

 You can determination of particle size according the standard method for gypsum soil with out soluble gypsum by using Hesse 1976 methods (particle size distribution in gypsic soils) 

Daer Diaa,

Thank you for your interesting answer.!