Now worth at least 10.6m euros (£8.8m), they were stolen from a collector's London home in 1970 and left on a train in Italy, with no indication of origin.
At a lost-property auction in 1975, the unsuspecting Fiat worker paid 45,000 Italian lire (23 euros; £19) for them.
He hung them in his Turin home before taking them to Sicily when he retired.
The worker only grew suspicious about their origins when his son saw another Gauguin in a book and noticed similarities with the painting in his father's kitchen.
The man consulted experts and police were eventually alerted.
The Gauguin painting, titled Fruits sur une table ou nature au petit chien (Fruits on a table or still life with a small dog), had been painted in 1889 and was thought to be worth between 10m and 30m euros (£8.3m-£24.8m), police said.
The Bonnard, La femme aux deux fauteuils (Woman with two armchairs), is valued at 600,000 euros (£500,000).
Cup of tea
According to a New York Times report published in June 1970, the two pieces were stolen from a property by Regent's Park in London by three men posing as burglar alarm engineers.
The men asked the housekeeper to make them a cup of tea. By the time she returned, the paintings had been taken from their frames and the men had made their escape.
Italian culture minister Dario Franceschini said: "It's an incredible story, an amazing recovery. A symbol of all the work which Italian art police have put in over the years behind the scenes."
Mariano Mossa, commander of Italy's heritage police, said he believed the paintings had been discarded on a train travelling from Paris to Turin after they were stolen.
"They were bought by an art-loving worker, who hung them for 40 years in his kitchen, first in Turin then in Sicily, after he retired," he added.
Gauguin was a post-impressionist master known for his creative relationship with Vincent van Gogh. His fellow Frenchman Pierre Bonnard is regarded as one of the greatest colourists of modern art.