The heap setting is located in the igateway.conf file in the /iTechnology folder.
Stop igateway first.
Open the igateway.conf file and search for JVM.
This brings you to the <JVMSettings> section:
<!-- <Properties name="prop-1">
Change the line:
To reflect the maximum heap size the implementation requires.
Although 256m is in most cases enough, this can be increased, but the suggestion is not to go over 1024m as this could start causing issues with the Java garbage collection.