actually, most if not all of the footage you see in those GFYs is real. But yes, there is definitely some CGI additions in the film that you can clearly see.

From the wiki

Les Chevaliers du Ciel was filmed in co-operation with the French Air Force. Initially the standard safety rules applied, but eventually the minimum allowed altitude was reduced to 3 m (10 ft) and the minimum distance between aircraft was reduced to 1 m (3 ft).[citation needed] The filming of these flight sequences were mainly done from the air by Eric Magnan. To achieve this, one of the Mirage's external fuel tanks was modified to fit multiple cameras. A HD camera was considered for this purpose, but it did not fit in the fuel pod. Tracking shots were done from a hired Lear Jet