Is Gsm antenna effect on microcontroller working?
There are 3 possible causes that I can think of (it's difficult to say more without further details):


SW bug: this is hard to confirm without actually finding the bug, but it might cause a lock-up and trigger a watchdog reset (do you have a watchdog running?) or a pointer error cause a jump to some memory location along the boot path (not so likely, I suspect)
Too tight power envelope: when your system tries to transmit, it draws more power than the power supply/battery can provide. This cause a temporary voltage drop, to the point that the system kinda power cycles. You can try to use a power supply with higher maximum current.
Electromagnetic interference: somehow the transmission interferes with the AVR. You can try to direct the antenna further away from the AVR and to increase the distance in between.


But you should really add more information to your question.