openHAB might be a bit more popular than Gatebox. We know about 1 link to it since March 2021 and only 1 link to Gatebox. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
I've seen some used ones on Ebay. You might try contacting the makers at their 'Contact Us' page on gatebox.ai. I don't understand Japanese so I can't navigate the Japanese version of their site, but supposedly that's where you can normally buy it. Source: almost 4 years ago
You can start on a general home automation like openhab.org. Source: almost 3 years ago
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ioBroker - flexible and modular application for the IoT and Smarthome
Josh.ai - Control and automate your home from anywhere
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