There's what we may call "a trend" in the Gulf area to hire only national employees. Do you agree? Do you think it is an effective hiring approach?

It would seem that, from the growing number of 'UAE Nationals only' jobs I have seen posted recently, that this is true.
However, it is only natural.
The demand for expat workers is/was only so that we may transfer our skills, experience and knowledge to nationals so that they may one day take the wheel and drive their own industries forward.
It has been and continues to be an effective policy to quickly grow national skill levels to the point where expats are no longer required. 
 

I am not GCC national, but I totally agree with this. It is there country they won't leave it if they are having a good job, and that is very important to the companies and the economy. If my country can not help me to get a job that I am qualified for, I will leave it, so my country loss, because no one is loyal than me to my country and its market,

But at the end any one should fulfill his position, does not matter where he is from.

Dear brother Rami , I would agree because every country  tries to provide job opportunities to their on citizens after they acquire the necessary experiences and necessary knowledge from  expatriates or hired foreign employees ( it may think of replacing foreigners by its own citizens ). For the second part of your question, my answer is : yes it is very much effective except for very complicated and highly sophisticated jobs which require high standard of knowledge and experience ( highly professional jobs ) .