Northside Hospital’s move to acquire Gainesville’s Northeast Georgia Diagnostic Clinic may be just its first major push into the Hall County health care market, said a national health care consultant familiar with Atlanta area medical practices. “Any hospital that buys a large medical group like (NGDC) does so with the purpose of establishing a beachhead in the market,” said Nate Kaufman, managing director of San Diego-based Kaufman Strategic Advisors.