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apt-get install php5-gd
simnle use 
# yum search php-

#Different Editing on php.ini
vim /etc/php5/apache2/php.ini
or
vim /etc/php5/cli/php.ini
or
vim /etc/php5/fpm/php.ini

#Add this extensions on your php.ini 
extension=gd.so

#Restart your Server
service apache2 restart
or
service nginx restart
service php5-fpm restart