The Gateways communicate over 2.4G back to your router, and this is the same wherever you are in the world. The frequency @aaiyar was referring to is the frequency that is used by the sensors to communicate back to the Gateway, this differs between regions, in much the same way that Z-Wave has different frequencies between regions.
@marktheknife also posted a very brief comment on this earlier in a separate conversation, explaining that sensors simply broadcast their readings and the Gateway receives it.
This is then limited to X sensors on each channel (sensor types). So if you had two Gateways that were for the same region, both would receive the readings, but would only be able to pair up to X sensors. From what @marktheknife and @aaiyar have explained, you would need a Gateway that operates on a different frequency and sensors to match that second frequency.
I haven't researched this myself, but this is what they were describing.
EDIT - See my post below linking to an EcoWitt forum post explaining some options / considerations for those wanting to run multiple Gateways / sensors that use the same frequency.
My post was putting that part of the situation to one side and commenting on the ability for two Gateways to still communicate with HE at the same time, a separate consideration to the issue of Gateway / sensor frequencies and sensor limits.