steps to create db link from oracle to MSSQL using gateway server

1.CREATE PUBLIC DATABASE LINK "DB_link_name"
   CONNECT TO "PTC_SSRSRMS" IDENTIFIED BY VALUES '06D0D'
   USING 'DB__name'

2.tnsnames.ora on oracle server oracledb_servername

DB__name=
  (DESCRIPTION=
    (ADDRESS= (PROTOCOL=TCP) (PORT=1621) (HOST=gateway_servername))
    (CONNECT_DATA= (SID=DB__name)) (HS=OK))


3.update on C:\Oracle\11g\dg4msql
initDB__name.ora

# This is a customized agent init file that contains the HS parameters
# that are needed for the Database Gateway for Microsoft SQL Server

#
# HS init parameters
#
HS_FDS_CONNECT_INFO=sqlserver name//sql db name
HS_FDS_TRACE_LEVEL=OFF
HS_FDS_RECOVERY_ACCOUNT=RECOVER
HS_FDS_RECOVERY_PWD=RECOVER

4.listener.ora
SID_DESC =
      (SID_NAME = DB__name)
      (ORACLE_HOME = c:\oracle\11g)
      (PROGRAM = dg4msql)
    )

5.tnsnames.ora
DB__name.WORLD =
  (DESCRIPTION=
    (ADDRESS= (PROTOCOL=TCP) (PORT=1621) (HOST=gateway_servername))
    (CONNECT_DATA= (SID=DB__name)) (HS=OK))