gw/acl_file is a SAP Parameter attribute which is used to control ACL (access control list) file of the gateway information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
This parameter specifies the name of an access control list (ACL) file.
With an ACL you can configure who is permitted to connect to the
gateway. The IP addresses of the clients form the basis of the file. The
same ACL file is used for the "standard" port and for the "SNC" port of
the gateway.
The syntax of the ACL file is described in
See SAP Parameter Documentation for full SAP documentation for this profile parameter.