the simplest way of getting to praia de gragoatá from rio's zona sul, would be by taking bus 7003 (also marked 740 d) that leaves from praça general osório in ipanema posto 8, across from the easy to spot zona sul supermarket.

the buses display the sign 'niterói / charitas' and are operated by the two companies auto viação 1001 and braso lisboa  - both lines follow an identical route. 

detailed bus itineraries for the two bus lines can be seen here and here - the 7003 bus also stops in copacabana along avenida nossa senhora de copacabana, at the bus stops marked brs3.