Mi Gateway is like a bridge that connect all Zigbee devices to the internet
Imagine if you got smart bulbs all using WiFi to connect to the internet. Your WiFi down means all cannot work
If got gateway, the devices connects via Zigbee so automations still can continue to run even if your WiFi is down.
This is just an example. Not all gateways connects via Zigbee. There are Bluetooth, Z-Wave, etc. Concept is the same.

I think they are 433 MHz but I don't have any hubs for it yet... or you mean the M2 Hub does support RF? 
D1 2 gang? or D1 wireless?

I using D1 switches no neutral

Ur right the range is the shittest of all. I have to shift my hub to middle of house to make sure all working. Based on aqara app, I have a door sensor and motion sensor right beside the D1 switch and both are showing acceptable zigbee signal, but the switch is showing 100% poor zigbee signal but I have a aqara cube 2 meter even further away from the switch and it have better signal 75% poor signal.

So meaning the aqara d1 switch is bad signal maybe because is no neutral so it must connect directly to the hub

Aqara D1 is Zigbee protocol, any of your switches between the hub and the furthest is neutral version?

Else it might be the limit of Zigbee, so you far my experience Wi-Fi most range… followed by Zigbee (assuming far from nearest hub/relay then shittiest is Bluetooth 😂.

Why dowan aqara app? The automation have proper options and runs locally, I used to use mihome too even the hacked version that can use China made with msia region for 3 months, then I switch to aqara app, better I feel

I’m using D1 with neutral (most switches around the house) and you would think all of them would have good coverage but… there’s a few switches that’s “poor” 😅 not every switch acts as relay. Only a few, and it changes depending on some “rules” I guess. Not sure how it works this Zigbee mesh thing.
Your best bet is to move the hub to middle, or add another hub 🤣

My motor blinds and curtain using 2 hub because of the limited RF signal range.