GRE

Hello, every university offer scholarships to international students on merit basis, there is no single criteria. Scholarships by Australian universities are however tough to get and they are not amongst the financial aid givers.

Dear Abishya,
A good GRE score is usually above 310. Though IELTS is also accepted, take TOEFL instead of IELTS if you want to study in US. A score of 100 in TOEFL is required. If you decide to take IELTS still an overall score above 6.5 is recommended. Please note that the admission requirements vary from university to university so you must visit the university websites to check the requirements before applying . Hope it helps.

IELTS and TOEFL are language proficiency exams and GRE is competitive exam for Masters in US or Germany.

Dear Steffi, if you check the websites of each of the universities carefully you will know that GRE is not mandatory for students having a prior degree in mathematics. Having said that since requirements and selectivity depends on a particular university alone I will strongly advise you to communicate with the universities directly.
Best Wishes

Dear Zeeshan, the actual conversion for the CGPA lies with the university & none would have the exact idea. Still you may self check it on the website for WES credential evaluation @ http://www.wes.org/students/igpacalc.asp.

Dear Steffi, 2 years is a pretty long time to study GRE & TOEFL as generally the students take 3 to 6 months to prepare & give the tests.
Since you are doing B.Sc which reflects your interest to pursue your Ms in USA BUT, to remind you that US universities would need a 16 years of education background or a US comparable degree to give you admission for their Ms courses.

you need both. IELTS. is a must to prove your English language test. GRE. if the University asks you have to do prove your engineering aptitude