Hi,
Are you referring to the full length LSAT prep tests, or just individual subject matter tests? The LSAT tests topics that are also on the GMAT (such as reading comprehension and critical reasoning), but it also tests a large amount of knowledge that is not directly relevant to the GMAT (such as logic games, which is about a third of the test, not including the essays).
I don't know that studying the preptests themselves would be a good preparation for the GMAT. The reason is that the preptests come with the correct answer key in the back, but no further explanation as to why one answer is correct or not. This will not help you decide why you got a question wrong.
I think you have the right idea in studying for the GMAT using LSAT materials, but I'd recommend you look into LSAT books that cover the topics, as opposed to full length LSAT practice tests. One good book that I've used in the past is the PowerScore Logical Reasoning Bible.
If you're still interested in the full length tests, you can buy them new from the LSAC website (lsac.org), and also probably used on sites like half.com. There are also books with a collection of multiple tests in one, which are more cost effective than buying individual tests.
Good luck in your studying