As I suspect you know from the phrasing of your question - Sylvia Plath gassed herself using a domestic gas oven. For further info., try here - http://www.sylviaplath.de
Bearing in mind she also tried to kill herself 10 years earlier by sleeping pills - she was a very troubled person, try this link - it may help you with the bigger picture http://www.sylviaplath.info/biography.html.
Watch the film Sylvia - it is really good. She was such a great writer/poet and her husband Ted hughes was a complete ba****d to her. He held her back and she finally crumbled (again) and gassed herself.