LSAT English is extremely difficult compared to GMAT.
That being said, GMAT>750 is not necessary for scoring in the 170's on the LSAT -- especially if the test taker is weak at maths, but good at verbal (which is rare among Chinese students.)
It really depends on individuals. To me, I prepared for it for about 3 months, practicing around 175, but ended up with a 171. One month after the LSAT, I took the GMAT with minimum preparation, and cracked a 750. The GMAT is totally one whole level below the LSAT.