How do graduate admissions committees in the US perceive grading systems other than GPA?
A general answer, not just for countries but even for schools in the US we're not familiar with, is that we don't know how to evaluate transcripts/grades.  Different places apply different standards, both within their curriculum, and for assigning grades.  A 77% on its own could be brilliant or horrible (and we mainly just care about grades in relevant classes).  One thing we'll sometimes do is check with other members of our department who might be familiar with the country/school's grading system and standards.

In any case, the main indicator we use for how talented and well prepared the student is are the letters of recommendation.  As long as you're getting letters from some professors who are known abroad for their research and/or are familiar with the system in the US, you should get helpful letters.