Did you see this forum topic?
I/O-only gateways are relatively light loads. You can approximate the workload of remote gateway connections (remote tag providers, remote history) as similar to Vision client connections, even when the front end is Perspective.
Visualization-only gateways only avoid the historization workload, so probably will need near the full forecast capacity.
As noted in the sizing guide and that topic, actual workloads vary greatly--you should test, test, test.