Gender did not exist as a mechanic in Gen I. The male/female symbols existed only in the species names for Nidoran-F and Nidoran-M (they won't show up if you type them here). In Gen II, most Pokémon could be male or female, with the exceptions of Legendary, Mythical, and object/artificial Pokémon. Some were exclusively male (ie. Tauros) and others exclusively female (ie. Jynx), while others were more likely to be male (ie. Machamp) or more likely to be female (ie. Jigglypuff) but could still be either.
However, gender is more than flavor text, it determines if two Pokémon of the same egg group can breed. The move Attract also makes use of it. Not only that, but how breeding works (the egg's species and inherited moves) depends on the gender of each parent. For example, Parasect is in the Bug and Grass egg groups, while Meganium is in the Grass and Monster egg groups. Since they share an egg group (Grass), they can breed. If Meganium is male (and it will be 87.5% of the time), the baby will be of the female's species (in this case, it'll be a Paras). The baby Paras will inherit the move Light Screen if the father Meganium has it, and that's a move it cannot learn through TM, level up, or tutoring.
Pokémon that are male only (Nidoran-M, Nidorino, Nidoking, Hitmonlee, Hitmonchan, Tauros, Tyrogue, Hitmontop) and Pokémon that don't have a known gender (Magnemite, Magneton, Voltorb, Electrode, Staryu, Starmie, Ditto, Porygon, Articuno, Zapdos, Moltres, Mewtwo, Mew, Unown, Porygon2, Raikou, Entei, Suicune, Lugia, Ho-Oh, Celebi) cannot produce eggs of their species unless bred with a Ditto. Baby Pokémon (Pichu, Cleffa, Igglybuff, Togepi, Tyrogue, Smoochum, Magby, Elekid), Mythicals (Mew, Celebi), and Legendaries (Articuno, Zapdos, Moltres, Mewtwo, Raikou, Entei, Suicune, Lugia, Ho-Oh) cannot breed whatsoever, and Ditto cannot breed with itself to make more Ditto.