Very similar to a Yummy mummy. ie a sexually attractive mother who looks after herself and her appearance.
There is, though, an important distinction. Gummy mummies are beautiful and attractive until they open their mouths. They have serious gum problems, normally caused by previous crack or meth addiction.
John: Wow, look at Tina's mum, she's a yummy mummy !!
Shaun: Who ? Shazza ? have you seen her when she opens her gob ? I'd say she's more like a gummy mummy.
coined the term