What is this instrument above the EGT in the Boeing 737?
This is a differential pressure and cabin altitude indicator.

The outer dial indicates the difference in pressure between the inside of the cabin and the outside in psi. During takeoff and landing, it must not be larger than 0.125 psi, as the note below the instrument says. If it exceeds 9.1 psi, the pressure relief valve will release the pressure to prevent damage to the airframe:


  Two pressure relief valves provide safety pressure relief by limiting the
  differential pressure to a maximum of 9.1 psi. A negative relief valve prevents
  external atmospheric pressure from exceeding internal cabin pressure.


(Boeing 737 NG FCOMv2 2 - Air Systems - Pressure Relief Valves)

The inner dial shows the cabin altitude in thousands of feet. During normal operations, it should not exceed 8,000 ft. This is a measure of the absolute pressure inside the cabin, converted to an altitude using the standard atmosphere (ISA) pressure profile.

Note: The instrument below is typically a cabin climb indicator showing the rate of change of cabin altitude in thousands of feet per minute:


(image cropped from here)

The EGT instrument in your image shows the Exhaust Gas Temperature of the APU and is typically installed further to the left on the overhead panel.