Having one GSM module - with subscription - for each slave would be expensive.

Drilling hole for wire from master to every slave would mean it would be quite simple to place the master where a GPS antenna cable can also be installed.

And it's easier to get enough signal to get time than it is to get enough signal to get a valid position.

I meant.
one GSM module at Master and distibuted to slaves cheaper than GPS.

Thank you
Jeckson S Ben