Let's talk gender tagging for FGO characters which canonically (read: gameplay mechanics) are registered as "Unknown Gender". Most of these are going to be pretty unambiguous, but I'm going to list them all below for reference.
Unknown Gender Havers
- Chevalier d'Eon: Arguably an IRL trans woman, FGO intentionally keeps it ambiguous. Tagged as 1other.
- Astolfo: World's most infamous otoko no ko. Tagged as 1boy.
- Enkidu: Canonically a shapeshifting pile of clay. Intentionally androgynous. Uses Shamhat's face, fanartists love making Gilkidu yaoi. Tagged as 1other unless otherwise indicated.
- Shi Huang Di: Ascended beyond gender as the first and only emperor of China in the alternate history. Shows off bare chest with pectorals & abs. Inconsistently tagged as both 1boy and 1other.
- Castor & Pollux: Inherent limitation of being a dual summon of mixed gender. Individually tagged appropriately.
- Caenis: Caeneus myth. Has booba. Tagged 1girl.
- Romulus Quirinus: Ascended beyond the need for gender or whatever. Gender is Rome(?). Romulus is unambiguously a dude. Tagged as 1boy.
- Ashiya Douman: Amalgamation of a couple of different evil spirits. Wears Hadanugi Dousa in the first few ascensions, with the nipples technically covered but it's still pretty clearly pectorals not breasts. Tagged as 1boy.
- Rengoku: Considered as such when manifesting as a Swimsuit Saber and in the Rengoku Alter costume dress. Shares Okita's voice actor as a swimsuit saber, voiced by Nakamura Yuuichi in the costume dress. Rengoku tagged as 1girl, Rengoku Alter tagged as 1other.
- Mysterious Ranmaru X: Servant universe shenanigans, not technically Mori Ranmaru or a genderswap of Mori Ranmaru. Has booba. Tagged as 1girl.
- Taira no Kagekiyo: Amalgamation of a couple of different evil spirits. Body is an aged up version of Ushiwakamaru. Has booba. Tagged as 1girl.
- Kiichi Hogen: Shapeshifting tengu. Has booba. Tagged 1girl.
- Kashin Koji: Sorcerer who keeps moving between new bodies. Can't remember their original gender. Depicted in a feminine body. Tagged as 1girl.
- Wandjina: Servant universe god shenanigans. Has booba. Tagged 1girl.
- Sugitani Zenjubou: Canonically the backstory is Sugitani had their gender changed via Sorcery after attempting to assassinate Oda Nobunaga. Has booba. Tagged as 1girl.
- Yamato Takeru: Never confirmed to identify with a specific gender in Fate/Samurai Remnant. Tagged as 1other.
nonbinary icon - Sirius: Pseudo-servant nonsense. Hibino Hibiki was originally a genderless artificial human (Triten); Sirius is using this as a base. Hibiki has boobs, Sirius doesn't seem to. Tagged as 1other.
TL;DR
Not really ambiguous (1boy): Astolfo, Castor, Romulus Quirinus
Not really ambiguous (1girl): Pollux, Rengoku, Mysterious Ranmaru X, Taira no Kagekiyo, Kiichi Hogen, Kashin Koji, Wandjina, Sugitani Zenjubou
Canonically ambiguous (1other): Chevalier d'Eon, Enkidu, Rengoku Alter, Yamato Takeru, Sirius
Currently undetermined (aka the main purpose of this thread): Shi Huang Di
Personally I think Shi Huang Di should be tagged as 1boy, since that's what they look like. Just wanted to make sure I put it out in the forums first (along with allowing people to voice their opinions on any other gender tagging while we're here) instead of just plowing ahead like I did last time with Enkidu.
On a slightly related note, people keep tagging pictures featuring Orion (Bear) as being 1other, which I'm pretty sure is like... against tagging policy? We don't tend to gender tag mascot creatures if humans are the main focus of the image, right?