YOU SAID:

put a against his gun head

INTO JAPANESE

彼の銃の頭に反対を置く

BACK INTO ENGLISH

Put the opposite on his gun head

INTO JAPANESE

彼の銃の頭に反対側を置く

BACK INTO ENGLISH

Put the other side on the head of his gun

INTO JAPANESE

彼の銃の頭に反対側を置きます

BACK INTO ENGLISH

Put the other side on the head of his gun