First log-in to your server through ssh access then follow this command or type these command in shell

wget http://ftp.sunet.se/pub/gnu/gmp/gmp-4.2.2.tar.bz2
tar -xjvf gmp-4.2.2.tar.bz2
cd gmp-4.2.2
./configure
make
make check
make install

nano /var/cpanel/easy/apache/rawopts/all_php5

add or type the following 

--with-gmp

then recompile PHP with default settings

/scripts/easyapache

save and compile, follow the instructions until the scripts finish recompiling the PHP. done!