Ok, thanks for the list of Symphonies in G major.
What it shows is that Haydn wrote proportionally more symphonies in G major than the other keys he used, however, apart from this it actually demonstrates what I said, there are very few symphonies (and other works) in G major: most of the people on this list are hardly household names, if you discount early works by Mozart, VW's London and Dvorak 8 there are hardly any*.
Well you could say that about a lot of keys, e.g. B-flat major if you discount Haydn and some early works by Mozart you have only Beethoven's 4th, Schumann's Spring, Dvorak's 2nd and Bruckner's 5th, A major only Mendelssohn's Italian, Beethoven's 7th and Bruckner's 6th, E major hardly anything... etc. For that matter, how many notable symphonies in C major are there between Schubert 9 and Sibelius 7? (Or after Sibelius 7 for that matter)
Beethoven has more piano sonatas in G major than any other key (4; E-flat major and C minor both have 3, other keys 2), two violin sonatas in G major (tied with A major), plus a string quartet, a piano concerto, two piano trios, a string trio, and various minor works. Schubert's last string quartet is in G major as is the D894 piano sonata. Chaikovsky, who rarely used major keys at all, used G major for his 2nd piano concerto and two of his four orchestral suites. Dvorak's Op.106 string quartet in G major should also not be overlooked; it's one of his finest. Brahms used G major for a string sextet, a string quintet and a violin sonata, all of which are among his best-loved chamber works.
In the post-tonal era we have notable symphonies in G from George Dyson and Lou Harrison, Walton's 2nd, as well as four symphonies that start in G and end in a different key: Mahler's 4th (ends in E major), Harris's 3rd (ends in G minor), Hindemith's Mathis der Maler (ends in D-flat major) and Nielsen's 6th (ends in B-flat major). Rubbra's piano concerto is in G as is Bartók's 2nd piano concerto and Stravinsky's Capriccio for piano and orchestra. I could go on, but I think you're already bored.
Certainly some composers did rarely use G major. I can only think of one complete work by Shostakovich in that key for instance, and none of Myaskovsky's 27 symphonies are in the key. But I think it's more about composers gravitating towards particular keys—e.g. Beethoven and C minor, Chopin and C-sharp minor, Bruckner and D minor, Janacek and D-flat major, Messiaen and F-sharp major etc. I think it just so happens that no one famous had a particular affinity for G major.
