OM_GM_CREATE is a standard SAP function module available within R/3 SAP systems depending on your version and release level. Below are the pattern details for this FM showing its interface including any import, export parameters etc and for your convenience any standard documentation available. All this information and more can be viewed if you enter the function module name OM_GM_CREATE into the relevant SAP transaction such as SE37 or SE80.
CALL FUNCTION 'OM_GM_CREATE' " * EXPORTING * transactionscode = SY-TCODE " sy-tcode IMPORTING next_transactioncode = " sy-tcode . " OM_GM_CREATE