Complex sentence: The Paris Air Show is primarily a commercial event, organised by the French aerospace industry's body the "Groupement des Industries Françaises Aéronautiques et Spatiales" (Gifas) whose main purpose is to demonstrate military and civilian aircraft to potential customers.
Split into simpler sentences: The Paris Air Show is organised by the French aerospace industry's primary representative body, the "Groupement des Industries Françaises Aéronautiques et Spatiales" (Gifas). It is a primarily commercial event, its main purpose being to demonstrate military and civilian aircraft to potential customers.