The photo, which the star posted to her Instagram account, shows Gavin, 48, holding a sleepy Apollo while the two touch noses. Gwen tagged the snap '#happyfathersday' and added the caption 'I always thought you'd be a good dad'.
Another tag on the post read 'simplekindoflife', a reference to No Doubt's 2000 single Simple Kind of Life, in which Gwen sang the lyric 'You seem like you'd be a good dad'.
Apollo is the 44-year-old Gwen's third child with Gavin, a British-born musician who fronts rock band Bush. The couple, who have been married for 11 years, already have two sons - seven-year-old Kingston and five-year-old Zuma.
We've previously seen Gwen carrying Apollo - dressed in an adorable mini Superman outfit - to watch middle son Zuma play football, but she kept the little boy's face cuddled close to her to shield him from the view of photographers.
It's going to be a busy few months for the ex-No Doubt singer. She already hinted that she was ready to cut her maternity leave short by making a surprise onstage appearance at Coachella alongside Pharrell Williams for a rendition of her 2004 hit Hollaback Girl, which he helped write.
Now she has taken on an even bigger commitment - joining the judging panel on the US version of The Voice, which began filming this week.
The show's producers are clearly overjoyed to see Gwen ready to get back to work, calling her 'the Queen of Awesome' as they celebrated her arrival on set by sharing a picture of her posing outside the NBC studios.