What are these "GENTEX" things on my walls?
It is the Gentex GX90 Remote Audible Signal which can be used as a fire alarm.

It probably is a fire alarm because I could only find references that mention use as a fire alarm, but I presume it could be used for something else.  I say that because they also offer a "Fire Alarm Red" color which would imply the off-white version could be used for something other than a fire alarm, e.g. tornado siren.