Female Space Marines were a misconception that many have taken to believe existed during the Rogue Trader era of Warhammer 40,000. The two official female models used as an example were introduced in 1987 and were named "Gabs" and "Jayne". They were advertised in White Dwarf n°99 and could be purchased for 5£. These were not female Space Marines, but were more proto Sisters of Battle, no different from the modern iteration of the female warriors clad in power armour and equipped with power swords and bolters.
It is unlikely that the sales of these two figures were indeed meant as Female Space Marines in the sense of "Female Adeptus Astartes", rather that they were generically designed armoured females which led to the development of the Sisters of Battle miniature line in the early 1990s, as a derivative of the all male Space Marines, not a part of them. The unpainted miniatures have "Sister" inscribed in the base, and the design appears to be a direct derivative of the "Sister Sin" character depicted in the "Sister Sin vs. Brother Vermillion" by Stephen Tappin, reproduced in the 'Rogue Trader: Warhammer 40,000' rulebook in 1987. Notably the breathing apparatus and shoulder pauldron design are recreated in these hand sculpted miniatures. Also, the Rogue Trader Rulebook even talks about the Adepta Sororitas.
The Sisters of Battle which developed, from this concept, are a unique unit of female warriors that have become a popular choice among Warhammer 40,000 players. The existence of Sisters of Battle makes female Space Marines obsolete in the official Warhammer 40,000 lore and miniature line. The impact these proto Sisters of Battle have is being propped up, mistakenly, to be an example of Female Space Marines with no other proof to back it up other than the fact that they wear Power Armor, wield Bolters and Power Swords all of which the modern iteration of Sisters of Battle are capable off. Nevertheless they remain iconic miniatures from Games Workshop's past, and has left a lasting impression on the Warhammer 40,000 community.