Gemmell has also appeared in the long running television show Silent Witness as mainly Detective Constable Kerry Cox. Gemmell appeared first in 1996 in eight episodes, following this stint as Cox Gemmell appeared in two other characters firstly as Beth Ashdown in 2006 and then finally in 2014 as Ellie Brooke. In the first series Gemmell appeared as Kerry Cox in "Buried Lies", "Long Days Short Nights", "Darkness Visible" and "Sins of The Fathers", as Ellie Brooke Gemmell appeared in "Undertone" and as Beth Ashdown in "Terminus".
Gemmell was married in Westminster, London, in 1997 to actor Ray Stevenson, whom she met in 1995 during the filming of TV drama Band of Gold. The couple divorced in 2005.
Gemmell has appeared three times in the BBC's police drama Waking the Dead, playing two different characters. Her first appearance was in 2002 in the episode Special Relationships as DI Jess Worral, a former lover of DSI Boyd. She next appeared in the episode Sins of seventh season in 2008 as Linda Cummings, an exceptionally intelligent serial killer. Gemmell reprised the role of Cummings in Endgame, the fourth episode of the eighth season of the show. The storyline had Cummings manipulating Boyd and revealed that Cummings' accomplice was responsible for the drugs overdose that killed Boyd's son Luke. The role reprisal of Cummings is a first in the show's history.
She performed in Macbeth and King Lear for the Royal Shakespeare Company in 2004/2005.
Gemmell has played a variety of roles mainly in theatres plus TV dramas. She played the leading female role Fever Pitch, based on Nick Hornby's novel of the same name, starring opposite Colin Firth and had another leading role in the comedy/drama 2 January (2006).
In August 2009, she starred as Rebecca Sands in two episodes of The Bill.
Gemmell starred in Episode 8 of Jimmy McGovern's BBC drama Moving On playing the role of Joanne, in November 2010.
In November 2011, Gemmell played Lady Shonagon in the adaptation for BBC Radio 4 Woman's Hour of "The Pillow Book", by Robert Forrest. She appeared as Jen, the wife of an adulterous civil servant, in Channel 4 drama Utopia, in early 2013.
Gemmell has also appeared in several shows a guest star, most notably Gemmell has featured in Casualty as Monica Darling during 2014. Gemmell has also appeared in long running series Midsomer Murders as Diana Carnarvon during 2015 in the episode "A Vintage Murder", the series was the third highest rating of the series.
In 2015, Gemmell appeared in the television series Penny Dreadful as Octavia Putney for five episodes.