What determines the gender of a Gen-1 Pokemon?

The Attack IVs.

From the Bulbapedia:

In Generation II, a Pokémon's gender was determined through its Attack IV. Due to this, a female Pokémon could never have a maximized Attack stat (unless they were a member of an all-female species, such as Smoochum), because female Pokémon were given the lower portion of the IV range. This also prevented female Pokémon of a species with a gender ratio of seven males to one female, such as starter Pokémon or Eevee, from being Shiny, due to that also being based partly on the Attack IV.

Basically, females were given the lower half Attack IV range and males the upper. This Reddit post goes into a brief discussion about it as well. I also found another post that explains the ratios:

87.5% male ratio: Attack IV needs to be 0 or 1 to be female all others result in a male

75% female ratio: Attack IV needs to be below 12 to be female

75% male ratio: Attack IV must be below 4 to be female

50-50 ratio: Attack IV must be below 8 to be female