Köster works in Germany as a comedian and actress. Following secondary school, Köster began training to become a nusery-school teacher which she broke off three weeks prior to completion. She then dedicated herself to music and painting, some of her paintings were also shown in exhibitions. To keep her head above water financially she also worked in a bar where, in 1987, she met Jürgen Becker. Becker asked her to write a few sketches for a radio programme. Gaby Köster thought the guest was crazy but still wrote a sketch for the programme. She was surprised when the text she had submitted was actually used on the German radio station WDR in the programme Unterhaltung am Wochenende, broadcast on Saturdays between 4 and 6 pm. Regular radio appearances and subsequently her own programmes were to follow.
Köster appeared on WDR television for the first time in 1991. Köster was a member of the cast of the alternative Cologne Carnival stage show between 1991 and 1995. Her somewhat bawdy humour is heightened by her Kölsch dialect.
Köster became known to a wider television audience through guest appearances on RTL Samstag Nacht. From 1996 until its discontinuation in 2005, she was a permanent cast member of the show 7 Tage, 7 Köpfe produced by Rudi Carrell. Between 1999 and 2003, she also played the title role of "Rita Kruse" in the comedy series Ritas Welt. One of her last appearances was in December 2007 in a show produced by ProSieben celebrating 20 years on stage of Michael Mittermeier.
At the beginning of 2008, Köster cancelled her Wer Sahne will, muss Kühe schütteln tour, which had only started a few weeks before. All other performance dates were later also cancelled. A media blackout prevented any information about Köster becoming available to the public. Her management took legal action against newspaper reports regarding an alleged illness.
It was revealed in early September 2011, that Köster had suffered a stroke  on 8 January 2008, the effects of which are still physically debilitating. She appeared on 7 September 2011 for the first time again following her stroke on the RTL Television programme Stern TV. Together with Till Hoheneder, Köster wrote the book Ein Schnupfen hätte auch gereicht – meine zweite Chance about her stroke and the time that followed. She also narrated the audio book version. During this period only Stern and Stern TV were allowed to publish up-to-date pictures of her.
Gaby Köster lives in Cologne and has a son from her former marriage with the director Thomas Köller.
- 1996: Gilden Kölsch Award
- 1998: Bambi in Category Publikumspreis Best Comedy Show for show 7 Tage, 7 Köpfe
- 1998: Goldener Löwe for 7 Tage, 7 Köpfe
- 1999: Deutscher Comedypreis in category best comedian
- 1999: Goldener Gong, tv award for team 7 Tage, 7 Köpfe
- 2000: Adolf-Grimme-Preis as comedian in Ritas Welt
- 2000: Deutscher Comedypreis for Ritas Welt
- 2000: Deutscher Fernsehpreis in category best actress serie
- 2003: Deutscher Comedypreis in category best actress in comedy
- 2007: Deutscher Comedypreis as best comedian
- Together with Till Hoheneder: Ein Schnupfen hätte auch gereicht – Meine zweite Chance. Scherz, Frankfurt am Main 2011, ISBN 978-3-502-15188-3
- "Drissdrecksdrisseliger Schlaganfall". Spiegel Online. 2011-09-06. Retrieved 2011-09-06.
- Medienbericht: Gaby Köster tritt 2008 nicht mehr auf Focus Online, 16. Februar 2008, retrieved, 11 December 2008.
- Gaby Köster: Verschollen hinter einer Mauer des Schweigens Focus Online, 29. Januar 2009, retrieved, 4 January 2011.
- Stern:Gaby Kösters Auftritt bei Stern TV:Ich habe keine Angst mehr vor dem Sterben (german)
- Erster TV-Auftritt nach Schlaganfall Stern.tv, retrieved, 8 September 2011.
- RP 7. September 2011