That's a good question. Typically, the ruleset implies that characters don't witness gates opening, they just are something you might come across.
This type of gate is different than Translating that characters can perform. A gate is open all the time, while characters do something that is more like teleportation, or folding themselves through 'space'. That is, you don't open a gate and go through it, but instead, meditate until suddenly you are somewhere else without having moved.
In this case, I may have been too subtle, but you're pretty sure the gate is operated by the aliens and their system terminal below.