From a 1990 G20 / 5.7L / 350
Working through some post rebuild issues and don't know what this sensor is and therefore whether it's part of the problem of not.
Seems part of the EVAP or Air/Fuel System
It's on the opposite side of the bracket as the MAP sensor atop the intake manifold.

 

From pictures in my 1988-98 Haynes C/K repair manual, I think it may be the EGR solenoid valve.

 

wow, thanks so much for the quick eye! You nailed it.