The attack occurred at the federal highway connecting Mexico City and Acapulco, the Guerrero state's Public Security Ministry said.
The criminals tried to block the road to the bus carrying 32 passengers, but the bus driver rammed their vehicle. The gang opened fire on the bus in response.
Those injured were hospitalized.
The Guerrero state has one of Mexico's highest crime rates. In 2014, 43 students disappeared in this state after participating in a protest against discriminatory hiring and funding practices in the city of Iguala. Initially, authorities blamed the alleged abduction on local drug cartels and criminal gangs, but additional evidence provided by independent experts pointed to local police and army having been involved in the kidnappings.