extremely thnakx to TiagoRibeiro and xpress_embedo for reply . And also sorry for late reply .
TiagoRibeiro, I checked baud rate and ground connection , both were fine . Basically the problem occurred for power supply . I changed the power supply to 9V 1A , then it is just working fine though it sends garbage value sometimes like earlier . But this time , the rate of sending garbage value is decreased .
And Congratulations TiagoRibeiro , you have won a great victory , hope it'll lead your team to the trophy ( Champion ):-D
xpress_embedo , my opinion is somewhat different , I also checked my gsm modem with virtual terminal of proteus , gsm modem sent the garbage values as exactly when it was connected with USART Terminal of Mikro C . I also read the values by Atmega 328P ( with lcd connected ) in proteus , I got exactly same garbage value which were shown in virtual terminal and USART terminal .
Though the rate is minimum than earlier , gsm modem still sends garbage value . If you know any other issues , please let me know .
Thanks again to both of you .