In case of outlook 2007 when you configure your gmail account with automatic pop3 configuration the problem of auto filtering of your mails received is done by outlook.
But this auto filtering most of the times does not function correctly which results in non junk mails or clean mails are mark as junk and outlook moves them automatically to junk folder.
You can simply fix the wrong filtering as follows:
1. Open Microsoft Outlook 2007
2. Open File Menu >> Tools >> select Options.
3. Now click on Junk Email Button and select the radio button which says: No Automatic Filtering
4. Click ok.
Now, manually move the existing non junk messages from junk emails to your inbox but for the new incoming messages outlook will not do any auto filtering.