I was wondering if someone could confirm if it is feasible to interface a GSM module for example SIM900, SIM900, Quectel's M95 or Neoway's N10 , maybe if we use the RS232 interface with HW the flow control pins RTS/DTR to the MAX485 or equivalent to talk to a master RS485 node ? or do i need to use a microcontroller to handle that job ?
Yes definitely. You do not need another micro controller for that.
Any product with UART can be brought out on RS485, with a level converter like this: