powering up the arduino and gsm shield module
Safe if >>

Yes it is safe. You have mentioned like 5-9vdc for GSM Module. Usually 
GSM module powered by on board voltage Regulator. So, i am guessing in your GSM board also having the voltage regulator which will take care of Constant Voltage. 

And also check your power supply whether it can able to provide 2.5A(1A+1.5A) with constant 5V. If there is no drop mean  no problem you can go ahead with this.

Not safe if >>

Some Power supplies voltage may drop to some certain level if you load to its full capacity. So check for the voltage drop at full load. If voltage drops for more than 0.7 obviously you can't use that power supply.