Why do I need to enter ICQ details in GSN configuration?
GSN uses ICQ to send the SMS messages. The reason for that is because it's free.
You don't need to have the ICQ client installed on your computer but you do need an ICQ account.
If you don't have one, you can't register right here: http://www.icq.com/register
Why am I being disconnected from my ICQ account when GSN is running?
When a new e-mail arrives, GSN logins to your ICQ account, and since ICQ doesn't allow two logins at the same time you're being disconnected.
To solve this issue, you can open another ICQ account for GSN.