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?