The Graduate Pharmacy Aptitude Test is an annual all India examination conducted by National Testing Agency on computer based test mode only, to evaluate candidates for admission of Pharmacy graduates into the Masters programme.

The eligibilty criteria for the GPAT exam is that the applicant must be a citizen of India and They must be Bachelor's degree holders in Pharmacy (4 years after 10+2, including lateral entry candidates). The final year students of of B. Pharmacy course are also eligible for appearing in GPAT exam.

So you cannot appear for the GPAT exam directly after completing your class 12th.