Yes, Reichu, I'm aware. I was there.
Worth keeping in mind that there are people reading the thread who probably weren't.
Even if you decide to take the (regrettably) popular route of using the more forced reading of the "contact experiment with the donor" line, Gendo simply being there at Antarctica to "make a donation" doesn't really account for all logistics, you know. Remember that he's not a scientist. According to the CI, he was an auditor for the research team. So, from the perspective of the many people who were actually studying Adam and setting up the contact experiment, Gendo would just be some penny-pinching asshole constantly looking over their shoulders and making sure they don't spend too much of Seele's precious money, more or less. Why in the world would the bloody book keeper
get to participate in the big experiment the team has spent months setting up? The research team would reserve the honor for one of their own, since scientists stick together and love using themselves in experiments besides. (Just look at those other two contact experiments.)
So you have to now make up some reason why Seele would force
the team to use the DNA of that obnoxious auditor in the contact experiment. But if Seele left the day before the experiment -- their lives are, after all, worth more than those of the lousy scientists they're utterly dependent on -- how would they even be able to enforce it? The team could just go, "lol, now that they're not here watching us we can do what we want! PARTY!" (Hey, contamination of samples happens. What are Seele gonna do about it?
Another problem is that if Gendo knowingly participated in an experiment that stuck his genes into a giant alien for undisclosed purposes that we don't know about but Gendo surely would, why does he seem to not know what Kaworu is until the guy deploys his ATF?* I suppose it's an open question either way, but especially so if Gendo took such a deeply personal interest in the procedure. "Welp, I'm going to just leave my DNA here, then completely forget I ever did such a thing and not give any shits what becomes of the experiment's results until Seele uses it against me 15 years from now. I'll also provide absolutely nothing resembling a hint that I have anything to do with this bishounen Angel -- not even the merest iota of a personal reaction! -- despite it ostensibly coming from my genes."
* (I guess you can claim that he DID know and just didn't tell Fuyutsuki, but then you have to explain what Gendo would gain from withholding such crucial information from his partner in crime.)