A classic, thoughtful writer of the Golden Age, Gallun is hard to compare. The haunting splendour of his best work is comparable to that of van Vogt, though perhaps more reflective. Anyhow, Gallun's contribution to the OSS is greater than van Vogt's because of the greater number of important OSS stories which Gallun has left us.
Harlei: I'd especially recommend Seeds of the Dusk and Old Faithful.
Zendexor: I'm going to read those two again, certainly, but the same goes for quite a few of the others, even the unwieldy Passport to Jupiter which I found a bit heavy going... it's nevertheless a powerful tale, one which I want to like. That "wanting to like" the second-rank tales is generated by those first-rank Gallun stories which require no effort to like: above all, Seeds of the Dusk.
That, in my opinion, is his best story. The best of many good ones (and indeed one of my all-time favourite science fiction stories), it is one of the few important attempts to portray intelligent plants, and it sheds an unforgettable light on both the far future Earth and the far future Mars.
Mercury: Passport to Jupiter (some scenes)
Moon: Magician of Dream Valley; Operation Pumice
Mars: Old Faithful; Seeds of the Dusk; Return of a Legend;
Passport to Jupiter (some references)
Asteroids: Red Shards on Ceres
Asteroid Progenitor Planet: see the interesting reference in Prodigal's Aura.
Jupiter: Passport to Jupiter
Io: The Lotus Engine and Invaders of the Forbidden Moon
Ganymede: Passport to Jupiter plus some references in Derelict.
Comets: Old Faithful
Raymond Z Gallun, "Davy Jones' Ambassador" (Astounding Stories, December 1935), "Derelict" (Astounding Stories, October 1935); "Invaders of the Forbidden Moon" (Planet Stories, Summer 1941); "The Lotus Engine" (Super Science Stories, March 1940), "Magician of Dream Valley" (Astounding Science Fiction, October 1938), "Old Faithful" (Astounding Stories, December 1934); "Operation Pumice" (Thrilling Wonder Stories, April 1949); "Passport to Jupiter" (Startling Stories, January 1951); "Prodigal's Aura" (Astounding, Astounding, April 1951); "Red Shards on Ceres" (Thrilling Wonder Stories, December 1937); "Return of a Legend" (Planet Stories, March 1952); "Seeds of the Dusk" (Astounding Science Fiction, June 1938)
For the "go for broke" attitude of Old Faithful, see Safety Last.
Red Planet colonists "going native" are discussed in The New Martians, regarding Gallun's 1952 tale, Return of a Legend.
For Operation Pumice see The Hidden Side of the Moon - 1949 - 1959.