Im not sure how it can be subjective when in battle, there is a clear winner and loser. The loser is the person who has been KOd/incapacitated/killed or in a situation where they are unable to continue the fight and the winner is the one who remains standing and is still capable of fighting.
the definition of suicide is very clear in that the person dies to kill another person/people at the same time and that befits the description of both RDS and C0 under normal circumstances because after the technique is used, they are killed. The 8 gates is different because instead of becoming the trigger for your own death and of someone elses simultaneously, you are using every bit of energy you body can muster to empower you with enough strength that can overcome virtually anything, though in the in the proccess, it consumes the bodies remaining energy and destroys it as well, which is why the 8th gate will guarantee death when the energy runs out completely.
The difference is that while the first two rely on the users life to initiate the technique, the aftereffect of gates is death. So in essence the 8th gate is considered a suicide technique in that it guarantees death afterwards, but not in the same fasion as other suicide techniques because their death is not required to invoke/use the technique.
And there're also draws where both characters get KO'ed as a consequence of the same battle.
The definition of suicide is that someone kills itself, period (it has nothing to do with fighting). What you're doing is considering the act of suicide as a valid mean to achieve a victory, something that i can't consider correct by my standards.
The last sentence is what caught my attention though. That's exactly where i wanted you to touch.
Death is very required to use the 8th gate since after closing the 8th gate Gai dies. Same as RDS where the user seals someone and dies after that same person is sealed.
The time argument is what's causing all this discussion.
As i said before, this isn't like a race to me. To determine a winner, the boundaries of a battle should extend to the consequences it has on those who participated in it. Gai isn't winning against anyone by using the 8th gate because even if he kills his opponent, he'll die as well as a consequence of that.
By that logic RDS earns wins as well since the user dies after someone else's soul is sealed first (effectively killing first).