His first official training came from 1887 to 1889 under François-Auguste Gevaert in composition and under Charles Duyck in harmony and counterpoint at the Brussels Conservatory, and in 1889, he was awarded the Belgian Prix de Rome for a cantata, Sinaï, which was very well received.
[1] A large orchestral work, La mer, which was first performed in Brussels on 20 March 1892, established Gilson as a national musical figure and also gave him success abroad, though not in Paris.
La Mer, the score which gave him his greatest success, is a set of four impressionistic movements ("symphonic sketches") in sonata form which were originally intended to illustrate verses by a French-speaking poet, Eddy Levis.
[1] Generally considered to form a programmatic symphony depicting the sea, Gilson's score (also known as De Zee) predated Claude Debussy's work of the same name by a decade.
[3] Despite being finely crafted, his later works such as the oratorio Francesca da Rimini tended to be somewhat conventional, lacking the originality displayed in the orchestration of La Mer.