GRE

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.

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

You have to appear for one of ielts or toefl to enter the boundaries of a country. This is mandatory. Gre is college admission in good ranked college. Not mandatory everywhere.

Dear Zeeshan, Congratulations for keeping a decent Academic profile. Yes there are options in US & UK for sure as there are a lot of universities available. For Germany, which is the hub of Mech Engg, you are expected to have better Academics besides GRE. You may refer to US News.com & DAAD for university shortlisting & check individual websites for more detailing.

There is no standard formula to calculate 10 point CGPA into 4 point CGPA. However most USA universities may accept standard maths conversion. You USA CGPA will be 2.68

obviously
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Thanks,
Abhishek