Gintama is my ultimate favourite when it comes to anime, and it's my biggest fandom overall after the GazettE. And that pretty much serves as justification for this exercise of the imagination. :D In other words, who would the members of the GazettE be in Gintama...?
VS
REITA
is obviously Gintoki Sakata. They are both seemingly laid-back and childish guys with very little motivation and difficulty taking anything seriously. But, inside they are really the ultimate gentlemen, who believe in defending their friends, resisting violence and living in harmony with everyone. Basically, they're two well-bred, boyish and gentle men, who can also turn pretty badass when they want to.
Also, there are some superficial similarities: Gin and Reita have a very similar way of speaking. Partly because of Tomokazu Sugita's (i.e. Gintoki's seiyuu's) similar diction, and partly because they use the same kind of language. Secondly, don't they sort of look alike, Gin-san and Reita?
URUHA is equally obviously Katsura, Gintoki's serious-minded childhood friend, who seems to inhabit a world of his own sometimes. Or all the times. Which reminds me of Uruha's "my pace" way of living - it's like he's oblivious to how other people view him and the world in general. They're also both quite well-groomed. (Although in my opinion Uruha's taste in clothes, especially private clothes, is somewhat questionable.) And didn't Uruha once have a plant as a pet? Well, Katsura has Elizabeth, who's... a peculiar species of alien.
^There they are both cross-dressing...
RUKI I'm casting as Takasugi, even though he's not exactly a psychopath bent on destroying the world. There's something similar about them. They are both very perceptive to the miseries of the world, artistic (Takasugi plays the shamisen, you know) and, let's face it, Ruki can be pretty scary when he wants to be. There's this melancholy that surrounds them much of the time. Also, they're both short, not to mention the existence of this photoshoot:
Compare with Takasugi: