LM25085A: About high-side gate resistor

Hi,

Could you tell me about LM25085A gate resistor?
Customer is trying to install gate resistor and snubbers to suppress ringing.

However,There is no explanation in application note or data sheet.
Is it possible to install a gate resistor in this device?
And I want to know the guidelines for resistor value.

Customer LM25085A use condition is：
-Vin=12.4 to 19.5V
-Vout=12V
-Iout=6.7A
-Fsw=500kHz to 600kHz
-PFET:TPCA8109
 http://toshiba.semicon-storage.com/ap-en/product/mosfet/detail.TPCA8109.html
-Diode:MBRB2535
-Inductor:6.8uH

Could you give me your advice?

Best Regards,
Yusuke/Japan Disty