I'm a big fan of using the LSAT as practice for GMAT CR and RC, partly because I think that the language on LSAT is tougher than that found in the GMAT OGs, and it's a great way to train yourself to battle your way through difficult passages. Unless you're superhuman, you'll be exhausted by the end of the GMAT verbal, and doing lots of LSAT practice can help you to develop your ability to stay focused when you're in a less-than-ideal frame of mind.
Osirus is exactly right, though--not all LSAT question types appear on the GMAT. I think that official LSAT material is much better than using a non-official test-prep book, but you don't need to lose too much sleep over some of the peripheral LSAT questions.
And if you buy just one LSAT book, I highly recommend The Official LSAT SuperPrep--it has great explanations, unlike many of the other official LSAT books.
