have a bit of supportive testimony from the Encyclopedia Britannica, 11th edition, published in 1911, which is considered a classic edition of that work. This is from an article on Gluck co-written by Sir Donald Francis Tovey, a well-respected musicologist and musician:
'When he [Gluck] was inspired there was no question that he was the first and greatest writer of dramatic music before Mozart. To begin with, he could invent sublime melodies; and his power of producing great musical effects by the simplest means was nothing short of Handelian. Moreover, in his peculiar sphere he deserves the title generally accorded to Haydn of "father of modern orchestration." '
-https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Gluck,_Christoph_Willibald
"In his peculiar sphere" somewhat diminishes the impact of the remainder of the sentence, especially as many ears might tend to wander back and forth between orchestral music and opera and, being ears rather than brains, might not erect a barrier between those two forms of music such that achievement is one field has little or no bearing on achievement in the other. In fact, some composers have even been known to work in both. Proving that Gluck produced mature Classical style orchestrations BEFORE Haydn may be more difficult, but I will give it a try. The decade of the 1760s is what I think needs to be examined.