YPOPs itself can't to the forwarding. YPOPs is a middle man program that lets a desktop mail program such as Microsoft Outlook, Outlook Express, or Mozilla Thunderbird download mail from your free Yahoo mail account to your computer.
What program are you using to download the Yahoo mail with POP3? This is the program that you created a new account in and pointed at the mail servers 127.0.0.1 in the YPOPs directions. In that program you will need to create a filter or rule to forward all mail it has downloaded to your Gmail account. If you let me know which program I can help create the filters/rules for the forwarding.
-Raymond