What does this symbol (three connected triangles) mean?
What do the symbol ,GDSN and GDSI , mean?


It looks like you have answered your own question. Your second image shows what the three triangles are. 

It's simply three cascaded logical inverters with an enable.

The driving force of each logical inverter gets stronger as you go to the right. In other words, the input may be very weak and the output will be very strong.

It's an inverting bufferer. 


  and what is its purpose?


The triple inverters are driving high capacitive loads, namely the gates of the two MOSFETs. If there wouldn't have been any triple inverter, then it would take too long to charge and discharge the gates of the MOSFETs. So this is clearly a design meant for high speed, or just low power during the time when the MOSFETs are opening/closing. 

The gate capacitance of the input to the triple inverter will be considerably smaller than the gate capacitance of the MOSFET pair.