gw/deallocate_timeout is a SAP Parameter attribute which is used to control timeout for deallocate and socket close 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 .
Time period in which the network connection for a DEALLOCATE must be
closed. If the connection is not closed within this time period, the
gateway closes the connection itself. This check is only made on
external connections (they appear as NORMAL_CLIENT in the gateway
monitor).
A parameter value of 0 deactivates the check
See SAP Parameter Documentation for full SAP documentation for this profile parameter.