Please have a look at page 1 of the MP3V5050 datasheet that illustrates possible package options.
The MP3V5050DP is a differential pressure sensor measuring the difference between pressures applied simultaneously to both inputs P1 and P2.
The MP3V5050GP/GC are gauge pressure sensors which also perform a differential measurements, but the pressure is applied to the P1 side and is measured against the ambient atmospheric pressure applied to the “P2 or vacuum” side through the vent hole in the chip. Whether to choose an axial (GP) or side (GC) ported package usually depends on application requirements.
Regarding the pin numbering, please look at Figure 1 of the datasheet. Vout is pin #4. With no pressure applied, the output voltage is around 0.12V as you can see in Table 3. At full-rated pressure (50kPa), the ouput voltage is typ. 2.8V.
The NC pins (1, 5, 6, 7, 8) have no internal connection and can be left open/unconnected.