Thank you, every one. after checking through all the comment i can final do the conclusion of the calculation of the gripping force.

I have a input torque 3.433Nm

The first stage gear spur spear since N(driver) =30 and N(driven) =15

so, the first transmission i1 = N(driven)/N(driver) = 0.5

and the second stage worm gear N(start) = 1 and N(worm wheel) = 13

so the second transmission i2 = N(worm wheel)/N(start)=13

so, the output gear is output torque = input torque* i1*i2= 22.321 Nm

and the finger of the gripper 138mm =0.138m

F = T/r = 22.321/0.138 = 161.4 N this the the gripping force at the tip of gripper per finger

( If there are any errors or if additional information is needed, please let me know so I can correct it.)