Be very careful with LSAT passages and questions: they are harder and longer than GMAT questions. Some of them are really good practice, but avoid the following:
Law passages -- this subject is not covered on the GMAT. Stick to the Humanities, Science, and Social Science subjects.
Questions that focus on a detailed understanding of one of the passage's key argument -- these questions are rarer on the GMAT and require a lot more careful reading on the LSAT, which can be the wrong habit on the GMAT.
Any question about a "principle" -- this is a common LSAT question type which you should avoid studying or worrying about because it does not appear on the GMAT.
Also be aware that LSAT passages are on average slightly longer than the long GMAT passages and that they have about 7 questions per passage as opposed to 3-4, so the timing is an entirely different animal.
The other test with similar style passages and questions, although mostly shorter passage lenghts, to the GMAT is actually the GRE; however, there are few good study materials for the GRE out there as it is. Nevertheless, that would be the first additional test resource that I'd look up after I'd exhausted the GMAT options.