This is a very interesting question indeed.
So, in order to add a system for monitoring, you'll need to have HTTP(S) connection to it (usually on port 443/SSL), so if you have Virtualization Extensions Service (main Veeam MP management point) installed in the same site as vCenter (for example on your SCOM Management Server), make sure VES has access to those 2 remote ESX hosts. It doesn't have to be collecting data remotely all the time - you can install a Collector on your Gateway server. In this case Collector will be connecting directly to these 2 hosts and then SCOM agent on a gateway will pick data and send it to SCOM. But for the initial contact, when you need to add a server in our UI, VES should have direct HTTP(S) access to the target, which means direct access to these remote ESX hosts.
If you don't have direct HTTP(S) connection to these remote ESX hosts, you may need to install a separate VES in the remote site, this means you will need two separate licenses, but I think our sales department can help you with that.
With respect to your Gateway, for simplicity, just think about it as a standard SCOM agent, if you install Collector on it it should work without any issues. The only question is HTTP(S) connection from VES to your servers as I mentioned above.
Let me know if you have any other questions