1. What is the maximum load the single instance of gateIn supports? I mean the number of concurrent requests supported by GateIn?
> Well, it definatly depends of the instance characteritics, I mean, it depends on hardware environement (or virtual hardware vram vcpu)
2. Is gateIn supports clustered environement?
3. Is there any server monitoring tool like Memory utilization, session management, each porrtal application memeory utilization as part of the out of the box.
> Yes, look at https://docs.jboss.org/author/display/GTNPORTAL35/Administration+and+Monitoring & http://docs.exoplatform.com/PLF35/index.jsp?topic=%2Forg.exoplatform.doc.35%2FGadgetUserguide.ManagementMOnitoringGadget.html
you can use other tools such as http://code.google.com/p/javamelody/ & you can also jmx connect your AS using jconsole, visualvm, or even cacti https://community.jboss.org/wiki/ConnectingJconsoleToJBossAS
4. How many documents can be accessed through users at a time? (no of document by no of users in 1 sec?)
5. CPU utilization under 1 lac users.
> As I said in 1. it depends on your hardware (or vhardware), node number etc...
hope it helps