I would have a separate management VM/appliance. While that has a higher up-front cost, it will provide better experience and save you time and energy down the road.
If you run gateway and management on the same appliance, the appliance will need to handle both tasks. R80.x Management requires significantly more resources than previous releases. While lower end appliances support it, the performance/experience may not be optimal.
If you move from a single gateway to a cluster, both cluster members will need to run management (a so-called Full HA config). If you add other gateways to the mix, you will need to acquire a management license to manage those other gateways anyway and take extra effort to migrate to a separate management appliance/VM.