What would be the most appropriate greeting for the gulf(arabic) nationalities though they speak english well ? Salam Walaikum or Good morning/ afternoon/ evening.

Its always better to greet people in language they are comfortable with.

As far as your question is concerned, Arab people likes to be greeted as "Salam Walekum"

ARAB PEOPLE WILL BE GREETED AS ASSALAMUALIKUM 

as the major of gulf citizens are muslimes the appropariate and preferred Greeting as salam walekum 

the best and appropriate greetings for the arabic peoples is Salam Walekum.

My answer is Salam Walaikum

Salam Walaikum is the correct answaer

Assalamualaikum Warehmatullahi wabarkatu

Assalamu  alaikum , wa Alaikumussalam

Asalama laikum...Walaikum alam