Management and staff of General Motors Corp.'s divisions are certain GM will centralize them at the marketer's new global headquarters in downtown Detroit, the Renaissance Center bought by GM last week. Executives inside and close to GM said they're concerned such a move would end their autonomy. That would give marketing czar Ronald Zarrella more power, they said, and he could speed GM's switch to brand management. Vice Chairman Harry Pearce said there has been no decision to consolidate marketing. GM's divisional agencies, all in suburban Detroit and Chicago, may have to move to downtown Detroit or open satellite offices.