I had the same error with also 2GB, I changed the run.conf.bat to set "JAVA_OPTS=-Xms256M -Xmx512M -XX:MaxPermSize=256M" and it started fine in 3'50".
I will reduce the shipped-in values (JBoss 5.1 default values are too low though)
Here is what I have observed.... (On Ubuntu 8 - Tomcat bundle)
If you do repeated start and stops of Gatein this causes the memory to be consumed. I think that somewhere things are not being cleaned up.
Also some comands such as ./gatein.sh stop -force .. the comand does not work as it does not identify a PID
Now since things are not cleaned up there are sometimes errors when redeploying portal.. work around is to fo to GateinHome/gatein/data and remove the directory.
this will alow you to redeploy the portal.war