Gmail allows users to setup external POP3 accounts in their Gmail accounts, however, the mail fetching does not happen instantly. Gmail schedules email download from POP3 at specific intervals.

However, you can now refresh ^[1] your POP3 account and fetch email on demand using a new Gmail Labs ^[2] feature called Refresh POP accounts. To enable this feature, head over to Settings -> Labs and enable the "Refresh POP accounts" feature.

Once you do that, you will see a new option to Refresh your emails, using this link you can refresh both your Gmail Inbox and also fetch emails from your POP accounts.