gw/max_overflow_size is a SAP Parameter attribute which is used to control Size of local memory area for SAP 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 size of the local storage area in which the
SAP Gateway manages the received requests.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.