GMAT score does not matter in PhD application even if you have 800
all that matters is your research experience
or let put this way, a guy with 400 GMAT & and a good research experience is undoubtedly a better applicant than a guy with 800 GMAT and zero research experience.
PhD application is not like MBA application, ,,,, as PhD applicant, you are applying for a job ( research / teaching assistant )
bottom line, if you have no research experience, then getting a high GMAT score will not make your application more appealing
لا الله الا الله, محمد رسول الله
You never fail until you stop trying ,,,