Gunnell, who was born in Chigwell, Essex, is the only British woman to hold four major track titles simultaneously – Olympic, World, European and Commonwealth. She will rename Ambience at a ceremony in Tilbury on 19 April.
Ambassador chief Christian Verhounig said Gunnell is a "wonderful advocate" for the 50-plus market.
"We’re incredibly proud to have Sally as our first ship’s godmother," Verhounig added. "Her ambitious drive and dedication to achievement echoes Ambassador’s determination for expansion.
"We cannot wait to welcome Sally onboard Ambience and celebrate this monumental milestone in Ambassador’s journey so far."
The 1,400-guest Ambience will enter service on 20 April with a maiden voyage to Hamburg, followed by a further 31 sailings calling at more than 90 different ports.
Itineraries will include the British Isles, the Norwegian fjords, Greenland, the Arctic and Iceland, plus several short break cruises.
Gunnell said: "Within a year, Ambassador has firmly established itself in the cruise industry and has a collection of brand values that resonate with me. From community and wellbeing, to being sustainable and ethical, we’re all responsible for the seas we sail on."