I’m looking into the ikea TRÅDFRI to use with my HomeKit setup but keep reading the gateway requires internet access. What does it need the internet for? Does it need an external service?
Yes, the gateway definitely needs internet access for Alexa and Google Assistant service integration, not sure if the Homekit support requires it as well.
Also to do the software updates to support new bulbs/sockets/devices.
The phone app should work with only a local network as it uses mDNS for discovery and CoAP over DTLS for communication.
But also worth pointing out that the gateway needs to be plugged into a ethernet socket as it is does not have WiFi.
EDIT: The following article implies that Homekit connections are all local (using a Apple TV/HomePod/iPad as a Homekit Hub for remote access)
The main thing for the gateway if you want to use it tradfri devices for Hue Bridge only is for firmware updates, for Homekit the Gateway is required though. You can use tradfri devices standalone without the gateway on Hue but no firmware updates. I can use my tradfri devices with Google Assistant and Amazon Alexa via Hue enabled functionality, no tradfri gateway is needed though. It's different for homekit than Hue because the app will provide you with a code for the homekit for adding the gateway to homekit. After the code entered you can control on the iPhone, apple tv or use Siri.