left click at gameserver.bat = >edit and see this :
REM -------------------------------------
REM Default parameters for a basic server.
java -Dfile.encoding=UTF-8 -Xmx512m com.l2jfree.gameserver.GameServer
REM
REM For debug purpose (for devs), use this :
REM java -Dfile.encoding=UTF-8 -Xmx512m -Xdebug -Xnoagent -Djava.compiler=NONE -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=7456 com.l2jfree.gameserver.GameServer
REM If you have a big server and lots of memory, you could experiment for example with
REM java -server -Dfile.encoding=UTF-8 -Xmx1536m -Xms1024m -Xmn512m -XX:PermSize=256m -XX:SurvivorRatio=8 -Xnoclassgc -XX:+AggressiveOpts
If you have free ram space then increase it :)