How to wire up GSM Modem(DCE) With MAX485
I do think is not possible without an intermediate protocol with MCU. The RE/DE is toggled with RTS from MCU, only when a master MCU requests data. Like PTT button on HAM radio transceiver. But the GSM module hasn't this feature, so a MCU is needed.

Other way is to use RS422, point to point (RS485 is for multidrop: 1 master + multi slave), then you have TX and RX signals for longer cable distance, If you do need additional control signals than you have to add those as well.

EDIT:

According to this link: Timer sequence with delayed off and long trigger pulse

I have made similar schematics. The TX signal is delayed, so that TX_EN sets the RS485 transceiver in transmit mode by RE/DE switch. 

When TX is high, the TX_EN becomes high at the same time as TX_delay. When TX goose low, it takes some time before TX_EN goes low (T=RC). This time should be adapted such, that is longer than the longest mark space duration of the TX UART signal - it is baudrate specific. 

The schmitt triggers come from 74HC14, but you could use also NAND gate schmitt trigger (74HC132), if they better arrange on PCB.







simulate this circuit &ndash; Schematic created using CircuitLab