The page @Kenster already posted is probably the best one for what you asked for: the way different races of people perceive him. (A second version at Wikicommons with better color.) Here's another octopoid form that hasn't been posted yet. (Mangaverse, I think.)
There are three side points, though.
First, the image of Galactus in any comic book defaults to humanoid even in the complete absence of humans. You could imagine that as an artifact of the comic/audience itself, converting a primordial reality into something they are able to comprehend. Really, that's nonsense and the idea of Galactus's mindbending was just one writer's idea. It hasn't been rigorously followed by others.
Second, Galactus himself is humanoid. (Super-Villain Classics: Galactus the Origin, No. 1) He was Galan of Taa in the last universe that crunched before creating this one.
He merged during the crunch with the 'Eternity' of the old universe, but both that entity and his duplicate in our present universe are nearly always depicted as an enormous humanoid form with stars and spacescapes within.
That 'true' shape of our universe's Galactus has appeared in Silver Surfer, Vol. III, No. 10. It's a brighter version of the same idea:
Third, so far there have been three separate Galactuses who've had their Galan aspect killed and their raw cosmic force processed into enormously powerful 'Galactus engines'. In each case, those machines maintained the form of severed heads &c. similar to the standard Galactus; none of them have been clouds, raw energy, or shifting substrates.