Heehee, yes, the friend is Okita, with his twisted sadistic desires, though Okita would just harass Hijikata directly rather than hire Gintoki to do it.
I like reversable sekes but usually end up with uke Gintoki in most situations. I have so many goddamn ships with this series.....Hijikata & Okita fighting over an uke Kondou, seme Okita harassing both Hijikata & Gintoki, Takasugi x Katsura, unrequited triangle of Elisabeth-Katsura-Gintoki, Takasugi hate-loving Gintoki, Takasuhgi & Gintoki fighting over Shoyo, Kamui x Gintoki, Tatsuma x any of the former samurai 4, & Gintoki chasing after every single guy in the series who has a goatee; Zenzou, Kondou, Madao/Taizou, & Takatin, plus the previous shogun, because he repeatedly "accidentally" sexually harasses all 4 of them in the anime. Lastly, the stuff that's cannon in the series; Gintoki was naked in bed with Kondou durring the Love Potion Arc, & Taizou told Gintoki that a very drunk Gintoki "was the one who bent him over."
I dunno why, but every time I read this I can't help but think that the seme looks a bit like Yu from Persona 4. ╮( ̄▽ ̄)╭ Yu has changed a lot since his high school days