Is there a way of running the R510 Unleashed in GW mode without having the DHCP service enabled? I already have a DHCP server in my portable training environment but I would like the R510 to act as AP and Gateway.
No, it is not possible to disable DHCP service on Unleashed Gateway AP. It is done this way to avoid config error . For example, in your own DHCP server if you configured gateway IP to be something other Gateway AP's LAN interface then traffic will not come to Gateway AP. This will certainly cause confusion and network won't work the way it is expected to work.
Since Gateway AP can provide IP addresses to all clients on the LAN side (both Wired and Wireless) why can't you disable the other DHCP server?
I expected this is likely going to be your reply. I (Product Manager for Unleashed) will take this as a feature request to allow advanced users like you to have the ability to disable it in the future.
For now, you could keep the Gateway AP off until all the clients get their IP addresses in your training environment and then turn back ON the Gateway AP. Another option is to reduce the DHCP pool size on Gateway AP to just '1' so that client devices will get IP from your DHCP server instead of Gateway AP.