There are various ways in which one can determine the ideal Syllabus for GRE. Some of them are
However as long as you take a proper diagnostic test, you may not get the exact idea of the syllabus. You can start with taking these diagnostic tests that help you identifying your weaker and stronger areas with respect to the syllabus for GRE.
Many GRE online preparation portals offer these test where you can get the results immediately after the test with feedback as well.
The important part is, as you start taking these test, you’ll also realize that there are different types of question as well. For example in Quant there’s multiple choice questions, Data entry questions, multiple answer multiple questions. Any student aiming for a HIGH GRE score should practise each type of these questions in plenty.
Once you practise these questions, you will have a good idea about the quant section. For verbal, there are 1500-2200 GRE words that you need to remember with meaning. You can again find these words in GRE online prep applications like Wordbot or Memrise. Adopting a clear learning methodology to master there 2000 odd words is a must.
There are various techniques like mnemonics, visual skills, root words etc. Post that, you need to start taking up sentence equivalence & text completion question which primarily require you to have a good grasp on the aforementioned vocabulary and pick up the contextual clues from the given question. The last but perceived to be the toughest, is the Reading comprehension questions, There are many Online GRE Preparation videos & tutorials available in youtube that you can use to solve this.
So to sum up, you can start by exploring many Online GRE preparation portals and start taking up, mock tests, GRE Style tests & full length tests to get the best idea on what the syllabus for GRE is. Hope that answers your question.