GSAM databases are sequentially organized databases designed to be compatible with MVS data sets. GSAM databases can be on a data set previously created or one later accessed by the MVS access methods VSAM or QSAM/BSAM. GSAM data sets can use fixed-length or variable-length records when VSAM is used, or fixed-length, variable-length or undefined-length records when QSAM/BSAM is used. If VSAM is used to process a GSAM database, the VSAM data set must be entry sequenced and on a DASD. If QSAM/BSAM is used, the physical sequential (DSORG=PS) data set can be on a DASD, tape, or unit record device. Because GSAM is designed to be compatible with MVS data sets, the GSAM database has no hierarchy, database records, segments, or keys.
GSAM databases are loaded in the order in which you present records to the load program. You cannot issue DLET and REPL calls against GSAM databases; however, you can issue ISRT calls after the database is loaded but only to add records to the end of the data set. Records are not randomly added to a GSAM data set.
GSAM database is basically used for Chekpointing/Restart. IMS takes cares of correct repositioning of the GSAM database at the time of restart.
So in order to use a sequential (MVS) file as GSAM file(IMS), you need to create DBD and have to make entry in PSB used for the program using GSMA file.