Answer 255 satisfies the condition so it's right but 128 is also satisfying condition so it is also a possible answer:
if we pick up 128 oranges.
on 1st gate he will give 64
on 2nd gate he will give 32
on 3rd gate he will give 16
on 4th gate he will give 8
on 4th gate he will give 4
on 6th gate he will give 2
on 7th gate he will give 1
64+32+16+8+4+2+1=127

finally 1 orange will remain with him.