There are no prerequisites for giving the GRE exam therefore you will be eligible. 

On the other hand, you do not generally need GRE scores in order to apply for MS in Germany.

The individual university requirements vary in German institutes and many of them do not require applicants to submit GRE scores. Instead, in many instances, transcripts for the applicant's previous education are sufficient in absence of GRE scores, although to be sure, you should check the application requirements for your preferred university in particular.

Exam scores which are necessary for admission in MS in Germany includes an english language proficiency examination such as TOEFL or IELTS . 

The general minimum score requirements for these are above 213 for computer based TOEFL, 550 for paper based TOEFL and 79 for TOEFL iBT. The minimum for IELTS is usually above 6.0.

In addition, students seeking admission in Germany are often required to submit scores for a German language proficiency test. These include TestDaF (German as a foreign language test) and DSH (German langugage test for the admission of foreign study applicants).

Hi. there is no any specific eligibility to take GRE for a post-graduate degree. Regarding your second question, you should have passed with at least 75% in your graduation or equivalent masters course in the same field and take up German language test. Some colleges might even ask for work experience.