"用石膏处理" can be translated into English as "treat with plaster" or "apply plaster for treatment".
"Treat with plaster" directly conveys the action of using plaster as a means of medical treatment.
"Apply plaster for treatment" specifies the action of applying plaster as part of a treatment process.
Both expressions are commonly used in medical contexts to describe the process of immobilizing a limb or body part using plaster for healing purposes.