Gac fruit is a tropical fruit grown in Vietnam as well parts of China and Southeast Asia. The fruit has both medicinal and culinary uses. In Vietnam the fruit is combined with glutinous rice to make a nutritious dessert (as xoi gac). Gac is exceedingly high in lycopene (even more so than tomatoes) and contains the highest levels of beta carotene of any know fruit or vegetable.
Gac fruit is very bland, almost flavorless. The tastes is closest to avocado without the nutty flavor.
If you can't find fresh gac fruit you can substitute:
From a medicinal perspective the gac fruit can be helpful in reducing atherosclerosis (hardening of the arteries), a remedy for dry eyes and has been tested for use in reducing the size of tumors.
The product is rarely seen in the U.S. There are online sources for buy the fresh fruit although it is very expensive. If you have a Vietnamese market in your area you can check the frozen foods department for frozen gac fruit puree.
Alternately you can try growing your own. Purchase gac fruit seed on Amazon.com: Gac Fruit Seeds.