gw/max_sys is a SAP Parameter attribute which is used to control Maximum number of gateway clients 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 maximum number of clients that can be
connected at a time. Note that the gateway regards an SAP application
server as one single client.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.