Gmail auto checks your POP3 accounts based on how frequently you receive email. To manually retrieve, you have to go to your settings, and under Check mail from other accounts, click - Check mail now.
Setup POP3
Gmail settings - cog wheel at the top right
See all settings
Accounts and Import
Check mail from other accounts
Add a mail account
Type in full email address
Next
Import emails from my other account (POP3) - already selected
Next
Fill out form
Username: full email
Password: provided to you
POP Server: mail.domain.com (already filled out)
Port: 995
Check the box - Always use a secure connection
Add Account
Send Mail As
Skip to step 4 if you just setup your POP3
Gmail settings - cog wheel at the top right
See all settings
Accounts and Import
Send mail as
Add another email address
Type the name you want to send as (they see as the sender)
Type your full email address
Leave checked - Treat as an alias
Next Step
Fill out form
SMTP Server: mail.domain.com (already filled out)
Port: 465
Username: full email
Password: provided to you
Secure connection using TLS (already selected)
Add Account
It will prompt you to confirm and will send you an email. You will probably be best to open a new tab, then go back to your gmail inbox. The email will likely appear there. If not, you can manually check now (see instructions at the top).