Personally I have been using LSAT tests to prepare for the Reading Comprehension and Logical Reasoning Parts of the GMAT. The RC is harder and so are the LR questions.
True, but there are LSAT logical reasoning
question types that just don't occur on the GMAT, like parallel reasoning (and flawed parallel reasoning), and questions like point of agreement (harder than it sounds) and method of reasoning. The reading comprehension passages are much longer than what you see on the GMAT as well. I think the material on the Official Guide and the Manhattan guides are sufficient for CR study.