Question about Zyxel GM4100

I forgot; the TV or telephony service is probably on a VLAN. You will lose this functionality.

Yes and no. It all depends on your configuration, your network skills and whether you are using PPP or IPoE/DHCP.

By default, this modem is configured in bridge mode.
In bridge mode, you'll need a router behind it; if you're using IPoE/DHCP, you'll need a DHCP client - in PPP mode, you'll need a PPPoE client.

If you don't know what I'm talking about, the Zyxel XMG3927 will be a better product for you.