The actress, who has also appeared in The Tudors, will be joined in the new series by Laura Fraser, Fiona Button, Emun Elliott, Heather Pearce, James Anthony Pearson, Tom Mannion and ex-Hollyoaks actress Roxanne McKee.
The six-part show, set in contemporary Glasgow, follows the sex lives and lover affairs of a group of twentysomething lesbians.
Gedmintas will plays Frankie, an irreverent and provocative photographer who is described as a 'commitment-phobe'. Fraser will play her former lover, Cat, who is still harbouring feelings for her, and Button plays Tess, Cat's best friend and flatmate.
"Harriet's scripts are very much authored pieces and, as such, the series takes a very vivid and authentic look at the lives and loves of modern British gay women on the cusp of life," said head of BBC drama Scotland Anne Mensah.
"The quality of the cast that the scripts have brought to the table really speaks for itself and Lip Service promises to be an emotionally-charged and utterly enjoyable drama series."
Lip Service is scheduled to air on BBC Three in spring 2010.