The Broadcom Corporation NetXtreme BCM5756ME Gigabit Ethernet PCI Express worked out of the box with the Tigon3 (tg3) driver. Your kernel should be configured as follows:

Device drivers  -->
  [*] Network device support  -->
      ...
      [*] Ethernet (1000 Mbit)  --->
          ...
          <*> Broadcom Tigon3 support

For Gentoo you should make sure that the symbolic link /etc/init.d/net.eth0 exists and points to /etc/init.d/net.lo.