Wow, you have lots of questions! (which is good!) 
... I will try to answer some of them here...
How do I check which machine is my sap ep installed on ? Which version is it and which service pack.
You can go to the URL http://<host>:<port> of your portal. On that landing page you can click on System Information and you can see lots of info about the server hardware, OS and the version you are running.
Also I would like to understand that if my EP is running too slow, what measures can I take to speed it up ?
Performance tuning is a big topic. I suggest that you take a look at this guide first Performance Best Practices Guide for SAP NetWeaver Portal 7.3
What is the meaning of full GCs when accessing the portal?
A full GC refers to the JVM doing a full Garbage Collection, they are costly. You can find more about GCs by reading any Java VM related help guides.
How do we check whether portal JVM is fully loaded in real memory?
Not sure what you mean here, maybe you are referring to how much paging the system has to do.
What is the meaning of having swapping on the OS level?
This relates the the question above, swapping or paging refers to when the OS needs to move things out of main memory and onto the disk (not enough space in main memory). If you have this issue it can slow things down and you should try to get more main memory. How much memory does your portal have?
Where can I check JVM parameter recommendations ?
See the guide above.
Where do I get GC.log file?
I think that would be in the JVM log folder, or at the root of the portal file system. Try doing a search on the server file system.
How do I check how many users are using my SAP EP ?
What version of the portal are you on? You can use portal activity reporting or the activity data collector, or you can even use a third party tool like ClickStream by Sweetlets.
Hope that helps. Good luck.
Simon