gw/max_overflow_usage is a SAP Parameter attribute which is used to control Percentage of overflow area 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 .
Specifies the usage of the overflow area as a percentage as of which the
gateway "slows down" its clients, that is, sends SYSN requests.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.