Read a lot, there is no shortcut. What I used to do is completed 5-6 only reading part of all the tests in one book, with each test my score was improving but it never got above 30 marks, as those were academic tests but the G test is quite easy exam. So your score will improve eventually.
There is one trick that I invented helped me a lot was, never read whole passage in one short. Read one para and then try to solve first 1-2 questions as questions are in order of passage, so keep answering questions and parallel to passage reading.