What Alex has said is correct, but just to clarify a bit more:
When you set up your domain, you will have entered some Name Servers (a minimum of 2 usually). These nameservers you entered (they usually look like ns1.example.com and ns2.example.com), they are your DNS servers.
You will need to configure your DNS servers to route your normal TCP/IP traffic to your commercial host, and your email traffic to your Google account.
This is easilly done.
In your DNS you will have a few types of records:
- A Record - this maps a name (domain/subdomain) to an IP Address
- CNAME Record - this maps a name to another A record
- MX - this maps your email to an A record
So! What you will need to do is:
- Set your A record to point to your commercial hosting provider (use their IP address)
- Your 'WWW' record should be a CNAME pointing to the A record, but if it is not, also update your WWW record to point to your commercial host.
- Set your MX record to point to the Google servers that they gave you
This way, all normal traffic will be routed to your commercial host, but your emails will still flow to your google account.