When an IP address is attached to a VM, a network interface is created and configured via DHCP, by means of udev.
Previously, the /etc/init.d/gandi-config script used to request this information at boot to later configure the system appropriately (for example in /etc/network/interfaces).
A static network configuration is written during the first boot of your server after creation (ex. /etc/network/interfaces for debian-based distributions), using the details provided in /gandi/config.
Gandi-provided system images will be able to set this information automatically and you'll be able to retrieve and edit them normally on each system.
For custom images or kernels, you'll need to configure this yourself (usually setting DHCP will allow you to get started).
Only the IP address is configured automatically in the case of secondary interfaces. All the other settings must be made by the user according to the intended setup.
For example, packets will continue to go out via eth0 unless the user configures the appropriate routes and rules on the system. The bandwidth quota and limitations of eth0 still apply, unless the destination is on a VLAN and the system can automatically route it through the right interface. You could therefore start by having low bandwidth interfaces, giving eth0 the maximum size to handle it all.
When an interface is detached from a VM, the information related to it is simply forgotten by the system.