Hand gestures

Millimeter-wave radiation is non-ionizing. It's just barely on the microwave side of the terahertz gap (the other side is infrared). While some effects on health are possible, the main concern is heating, which can cause problems for some bodily tissues such as the retina. But a sensor should not be operating at an amplitude where that is likely to become a concern. mm-wave does not propagate (in atmosphere) farther than a few kilometers, and won't go through walls (or most other obstructions) at all, so "spillage" from your neighbors is rather unlikely to make a difference.

In short: It's mostly harmless.

(Disclaimer: I work for Google, but I had nothing to do with Pixel.)