What can you do to supplement low grades for admission into masters programs that don't use GRE or GMAT?
As far as I know, such programs will consider your GPA, GRE (general and/or subject test) or some equivalent tests, letters of recommendation, research experience and personal statement, or a subset of this list. Thus, it is really more precise to say that all of these factors supplement each other, rather than simply saying that only GRE and GPA supplement each other. Unless your GPA is lower than the cut-off line, which some programs impose, then it follows that good letters of recommendation, research experience and personal statement will help with your application. This will especially be true if you have exceptional academic achievement that is not reflected by the GPA, in which case the admission committee will likely overlook the low GPA.