Based on who Equius is as a character and his belief in the hemospectrum, it's more likely that Equius was smiling because Gamzee was finally acting like a highblood. Equius' deal with orders is more about the hierarchy and order than it is about any kind of fetish. Equius obsessed over Gamzee's behavior because he acts outside the norm for what someone have such a high caste is supposed to. So his smile likely stems from Gamzee finally, you know, doing what he's supposed to do.

You said that a lot better than I did

~Mod T