Basically Gintoki is either a Master or a Servant. If he's summoned by the Fate Grand Order protagonist then have him have a Fake Noble Phantasm for a while. His real Noble Phantasm will be up to you and give him E rank charisma but in actuality it's EX rank charisma. If he's summoned by someone in the original like Illya, or in Fate Extra by Hakunon, maybe even in Fate Apocryphia. Or he can be a Master and replaces Shrou and becomes something akin to a Magus Killer or something and gains the title Shiroyasha that way. Or somehow the Holy Grail War starts in Gintama Edo and Gintoki summons a Servant. It has to be a Japanese Servant and female like. Sakura Saber(that would be interesting right), Miyamoto Musashi, or Kato Danzo. That's all try to make it more serious no crack. Also no Yaoi or Yuri.