The Graduate Record Examinations is a standardized test for admission requirement for many graduate schools in the United States and Canada. The guidelines for the preparation is available on the official site of ETS which can be used by the candidates who want to prepare themselves for the exam. Sample questions are available in the official website

https://www.ets.org/gre/revised_general/prepare/quantitative_reasoning/multiple_choice_one/sample_questions

Also the syllabus, preparation materials , details of exam etc are available in the official site

https://www.ets.org/gre/

You can buy the practise books that are available online or can use the practise books that are available in official site

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.ets.org/s/gre/pdf/practice_book_GRE_pb_revised_general_test.pdf&ved=2ahUKEwiX3sn8mJ3sAhXvzzgGHf3KDU8QFjAAegQIGBAC&usg=AOvVaw3imJQNj_VgsDE_mQYSNP2a&cshid=1601892098319

Some books are

• Reading Comprehension & Essays GRE Strategy Guide by Manhattan Prep
• Essential Words for the GRE by Philip Geer
• GRE Prep by Magoosh ,book by Chris Lele and Mike Mcgarry
• Fractions, Decimals, & Percents Gmat Preparation Guide ,bo ok by Manhattan Prep etc