Likely playful, IMO. Cats play pretty roughly with each other.
With my two, I can tell when they're playing versus serious by their sounds; when they actually get angry with each other, there's quite a bit of hissing, growling, and fussing, but when they play they're mostly quiet. Their play is just as ferocious as their squabbles. When they do vocalize or hiss during play, it means "You're going too far now" and the other cat knows to back off.
Personally I would not separate them. The kitten is holding its own, no one seems to be getting bullied, and they don't seem too upset. Cats need to work out their own dominance, and sometimes it looks very rough by human standards - but as long as there are no injuries, it's probably okay.
It's possible your kitten is annoying Gosha, but that's actually not necessarily a problem - when Gosha responds negatively to something, the kitten learns appropriate boundaries. Cats can actually do a pretty good job of teaching each other to play nicely.
The Boys: Maisie, Zephyr, and Cullen.