vRack servers communicating out via

I have a set of VMs running under XenServer on OVH dedicated servers.

I'm currently using a Firewall which plugs into my vRack and acts as NAT to servers in the vRack and is also the default gateway. It sits in front of servers in a local address range 172.16.x.x

However I also have a public RIPE block which has it's own gateway IP.

What I would like to do is not use the firewall any more (as it's old) and use the RIPE block for incoming traffic which then gets sent by my load balancer via another network interface to the private address range - this all works.

However the problem comes in configuring the VMs on the private address range so that they can communicate out (presumably via the gateway IP on the RIPE block), at the moment they use the internal address of the firewall as the gateway but that won't work as I want to switch it off.

e.g. I have a VM running on 172.16.1.1 currently it's gateway is 172.17.0.254 - which is the local address of the firewall, which I want to remove - but since the VM doesn't have an address in the RIPE range it can't use the RIPE gateway?

tl;dr what should I use as the gateway on a vrack?